AT LEAST five people are dead and more than 1,000 people have been rescued as Tropical Storm Harvey pummelled the US Gulf Coast with catastrophic rain.
Dozens of injuries have also been reported.
The US National Weather Service (NWS) issued a dire warning on Sunday as massive floods inundated Houston in the aftermath of Tropical Storm Harvey, saying the severe weather conditions were "beyond anything experienced."
"This event is unprecedented & all impacts are unknown & beyond anything experienced. Follow orders from officials to ensure safety. #Harvey," it said on Twitter.
The NWS said some parts of Houston and just west of the city may receive a Texas record of 1270 millimetres of rain as the storm, downgraded from a hurricane, stalls over Texas.
NWS meteorologist Patrick Burke said rainfall totals will end up around 1016 millimetres or more for Houston on average.
Mr Burkes said: "We're in kind of unprecedented territory with this storm."
Helicopters, boats and high-water vehicles swarmed around inundated Houston neighborhoods, pulling people from their homes or from the turbid water, which was high enough in some places to gush into second floors.
The flooding was so widespread that authorities had trouble pinpointing the worst areas. They urged people to get on top of their homes to avoid becoming trapped in attics and to wave sheets or towels to draw attention to their location.
Harris County officials asked the public to contribute boats and high-water vehicles, which they said were "desperately" needed to help rescue people before nightfall.
At a news conference on Sunday, County Judge Ed Emmett said that "government assets are fully utilized," and that boats from outside the area could not get there.
FLOODS FORCE CLOSURE OF HOSPITAL, AIRPORTS
Judge Emmett also said that Ben Taub Hospital, the county's public hospital, is being evacuated because flooding problems in the basement are disrupting power service.
Judge Emmett overseas government operations in Harris County where Houston is located. He told a news conference that evacuated patients are being taken to other area hospitals. It was not immediately known how many patients were being moved.
Meanwhile, both major airports in Houston have been closed.
A Houston Airport System statement at midday on Sunday said George Bush Intercontinental Airport and Hobby Airport are closed to commercial flights until further notice.
Officials say roads in and out of both airports are shut down due to flooding.
The storm also forced a Houston television station to evacuate to higher ground as rising floodwater overtook their offices.
Reporters and editors at KHOU on Sunday tweeted photos and videos of flooding inside the station's lobby.
Rescuers answered hundreds of desperate calls for help on Sunday as floodwaters from the remnants of Hurricane Harvey climbed high enough to begin filling second-story homes, and authorities urged stranded families to seek refuge on their rooftops.
HOUSTON MAYOR WARNS AGAINST EVACUATION
Meanwhile, Houston Mayor Sylvester Turner said authorities had received more than 2,000 calls for help and would be opening the city's main convention center as a shelter. He urged drivers to stay off flooded roads to avoid adding to the number of those stranded.
"I don't need to tell anyone this is a very, very serious and unprecedented storm," Turner told a news conference. "We have several hundred structural flooding reports. We expect that number to rise pretty dramatically."
The mayor defended his decision not to ask residents to evacuate before the heavy rain from Harvey swamped roads and neighborhoods across the nation's fourth-largest city. He said there was no way to know which neighborhoods would be most vulnerable.
"If you think the situation right now is bad and you give an order to evacuate, you are creating a nightmare," he said, citing the risks of sending the city's 2.3 million inhabitants onto the highways at the same time.
Areas south of Houston appeared to be at greatest risk. Some flooding was reported in downtown Houston and in the Texas Medical Centre.
CNN reported that more than 1,000 people were rescued overnight from record flooding in the city.
RECOVERY OPERATION 'WILL TAKE YEARS'
The head of the US Federal Emergency Management Agency (FEMA) told the network his agency is already gearing up for a years-long effort to help Texas recover from the damage wrought by the storm.
"FEMA is going to be there for years," administrator Brock Long said Sunday on CNN's "State of the Union."
He added: "This disaster is going to be a landmark event."
Harris County Sheriff Ed Gonzalez used Twitter to field calls for assistance for those trapped inside homes, attics and vehicles.
Among those seeking help was a woman who posted: "I have 2 children with me and the water is swallowing us up."
Officials urged people not to crawl into attics but to get on top of them. Police evacuated two apartment complexes overnight in the Greenspoint neighbourhood, rescuing more than 50 children from rising water.
"It breaks your heart," Houston Police Chief Art Acevedo said as he stood in waist-high water during a lifestream post on Twitter. "But it's Texas. We'll get through it."
Rainfall of more than four inches per hour resulted in water levels higher than in any recent floods and higher than during Tropical Storm Allison in June 2001, said Jeff Linder of flood control district in Harris County, which includes Houston.
In Friendswood near Houston, authorities asked people with flat-bottomed airboats or fuel for them to help rescue people from flooded homes, KPRC-TV in Houston reported Sunday morning.
Houston officials used dump trucks and city buses to move residents to higher ground.
The rescues unfolded a day after the hurricane settled over the Texas coastline.
US National Weather Service said on Sunday morning that at least five people have died in the region.
Anxiety ran high throughout the region between Corpus Christi and Houston because some of the areas with the greatest hurricane damage were inaccessible to rescuers. And the forecast for days of steady rain threatened to inundate the region's flat landscape with as much as 100 centimetres.
TRUMP PLANS TEXAS TRIP
Major rescue operations underway!— Donald J. Trump (@realDonaldTrump) August 27, 2017
US President Donald Trump tweeted on Sunday morning that he was chairing a cabinet meeting about the crisis.
Mr Trump tweeted: "Going to a Cabinet Meeting (teleconference) at 11:00 A.M. on #Harvey. Even experts have said they've never seen one like this!"
In between other tweets slamming US trade deals and about his proposed wall along the Mexican border Mr Trump quoted experts saying the event was a "a once in 500 year flood".
Wow - Now experts are calling #Harvey a once in 500 year flood! We have an all out effort going, and going well!— Donald J. Trump (@realDonaldTrump) August 27, 2017
He also tweeted on Sunday morning that he would visit Texas. "I will be going to Texas as soon as that trip can be made without causing disruption," the president posted on Twitter. "The focus must be life and safety."
In the island community of Port Aransas, population 3,800, officials were unable to fully survey the town because of "massive" damage. Police and heavy equipment had only made it into the northernmost street.
"I can tell you I have a very bad feeling and that's about it," said Mayor Charles Bujan, who had called for a mandatory evacuation but did not know how many heeded the order.
Some of the worst damage appeared to be in Rockport, a coastal city of about 10,000 that was directly in the storm's path.
The mayor said his community took a blow "right on the nose" that left "widespread devastation," including homes, businesses and schools that were heavily damaged. Some structures were destroyed.
Rockport's roads were a mess of toppled power poles. A trailer blocked much of one major intersection. Wood framing from ripped-apart houses was strewn along Route 35 on the town's southern end.
Harvey's relentless wind tore the metal sides off the high school gym and twisted the steel door frame of its auditorium.
"We're still in the very infancy stage of getting this recovery started," said Aransas County spokesman Larry Sinclair.
One person was killed in Aransas County when in a fire at home during the storm, county Judge C.H. "Burt" Mills Jr. said.
Another person - a woman who tried to get out of her vehicle in high water - died in flooding in Harris County, where Houston is located, though authorities had not confirmed a cause of death, said Gary Norman, a spokesman for the Houston emergency operations centre.
As many as 14 people suffered minor injuries, including slips and falls, scrapes and a broken leg, Mills said.
About 300,000 customers were without power statewide. Governor Greg Abbott said it would probably be several days before electricity is restored. Meanwhile, the storm was barely moving. Rainfall totals varied across the region, with Galveston receiving around 20 centimetres, Houston 28 centimetres and Aransas 25 centimetres. Tiny Austwell got 38 centimetres.
In Houston, authorities pleaded with people not to leave their homes as a flood emergency was declared.
"The streets are treacherous," Houston Mayor Sylvester Turner said. Elsewhere in the storm's immediate aftermath, the Coast Guard had rescued 20 people from boats and barges in distress, said Captain Tony Hahn, commander of the Corpus Christi sector.
The Corpus Christi port was closed with extensive damage. Because the city is the third-largest petrochemical port in the nation, the agency will be on the lookout for spills, Captain Hahn said.
The fiercest hurricane to hit the US in more than a decade came ashore late on Friday about 48km northeast of Corpus Christi as a mammoth Category 4 storm with 209km per hour winds.
Harvey weakened to a tropical storm by midday on Saturday.
By 7am on Sunday, Harvey was centred about 100km southeast of San Antonio, with maximum sustained winds of about 72.42km per hour, according to the National Hurricane Center, which described the flooding as "catastrophic."
Harvey came ashore as the fiercest hurricane to hit the US in 13 years and the strongest to strike Texas since 1961's Hurricane Carla, the most powerful Texas hurricane on record.
The storm's approach sent tens of thousands of people fleeing inland. Just hours before landfall, the governor and Houston leaders issued conflicting statements on evacuation.
The governor urged more people to flee, but Houston officials recommended no widespread evacuations, citing greater danger in having people on roads that could flood and the fact that the hurricane was not taking direct aim at the city.
The last Category 4 storm to hit the US was Hurricane Charley in August 2004 in Florida.
Update your news preferences and get the latest news delivered to your inbox.