Houston - Rescue teams in boats, trucks and helicopters scrambled on Monday to reach hundreds of Texans marooned on flooded streets in and around the city of Houston before monster storm Harvey returns.
Houston mayor Sylvester Turner said more than 2 000 people had been brought, soaking and desperate to rapidly-filling shelters in America's fourth largest city - and more are yet to come.
The 911 emergency services line has received more than 56 000 calls, but city officials urged residents facing life-threatening storm water floods to remain on the line and trust that help will come.
"The goal is rescue, that's the major focus of the day," Turner said, flanked by grim-faced city officials at a televised press conference, warning that 185 "critical rescue missions" are still pending and many more are expected.
"People are needing clothing. Literally, people are coming in and they are coming in wet," he said. "We have kids, babies, up to senior citizens. They are needing everything. They are needing clothing, food, medical supplies."
Houston fire chief Samuel Pena urged patience, promising: "We fully recognise there are many other people out there in distress situations and we intend to get to every one of them."
Harvey hit Texas on Friday as a Category Four hurricane, tearing down homes and businesses on the Gulf coast before dumping what meteorologists said was an "unprecedented" rainfall inland.
The Texas bayou and coastal prairie rapidly flooded, but the region's sprawling cities - where drainage is slower -- were worst hit, with highways swamped and street after street of housing rapidly rendered uninhabitable, power lines cut and dams overflowing.
Robert Dressell and Adam Turner of Tyler pack the back of a pickup truck with fuel heading to Houston. (Sarah A Miller, Tyler Morning Telegraph via AP)
The US Army Corps of Engineers on Monday moved to progressively open the Addicks and Barker dams - under pressure from what the agency has dubbed a "thousand-year flood event" - to prevent a catastrophe on the outskirts of Houston.
At least three people have been confirmed dead so far, but that toll is expected to rise, and the disaster is far from over: Harvey has turned back on itself and is hovering on the Gulf coast, sucking up more rain and threatening a new landfall on Wednesday.
Flood-hit Texas is braced for a renewed battering and on Monday President Donald Trump - facing the first major natural disaster of his presidency - pre-emptively declared a state of emergency in neighbouring Louisiana, freeing up federal funds.
Trump plans to go to the disaster zone on Tuesday and aides said he has been constantly in touch with federal and state officials involved in the emergency response.
Federal Emergency Management Agency (FEMA) administrator Brock Long said finding shelter for those flooded out of their homes would be his next priority.
"This is a landmark event. We have not seen an event like this. You could not dream this forecast up. You couldn't draw this situation up," he warned.
"It's crazy to see the roads you're driving on every day just completely under water," Houston resident John Travis told AFP.
Wilford Martinez grabs the median as he is rescued from his flooded car. (David J Phillip, AP)
While the disaster response is now focused on the huge city, centre of a fast-growing but loosely-planned urban area now home to more than six million people, many of those living in smaller communities by the coast were also driven from their homes.
Robert Frazier, 54-year-old foreman mechanic, left his home in La Porte, south of Houston, with his wife Judy on Sunday morning and made it as far as a motel in Hankamer on the road towards Louisiana and still in Harvey's path.
"We're trapped," Frazier said, speaking to AFP after he had tried to return home for some of his abandoned possessions, but finding the highway cut.
"I haven't been through anything like this. So I thought it was just going to be a little rain and a little high wind, but it turned out to be a lot of rain. I never expected floods like that," he said.
His wife Judy said she could only pray the rain would stop, after leaving home with just two sets of clothes, their medicine and their dog. "The water was coming up over the road, so we just decided to get out while we could," she said.
The NWS said that between June 1 and Sunday, Houston had received 117cm of rain - almost as much as it would expect in a year - in only three months.
"The breadth and intensity of this rainfall are beyond anything experienced before," the National Weather Service said, as the storm spawned tornadoes and lashed east and central Texas with torrential rains.
Houston's George Bush Intercontinental Airport and Hobby International, the city's two airports, stopped all commercial flights.
Thousands of National Guard troops joined local police and emergency workers to help with rescues in inundated areas of Houston.
Joe Garcia carries his dog from his flooded home in Spring, Texas. (David J Phillip, AP)
Boats also were being deployed, but more were needed. Harris County Judge Ed Emmett appealed to residents to use their own vessels.
"I'm not even thinking about myself right now," Bryan Curtis, who came to Houston with his jet skis to rescue people, told AFP. "I'm here to help, I want to do my part."
FEMA said there should be no illusions about the long-term impact of Harvey. Long told CNN it would take "years" to recover.
Coastal Texas is home to a large number of oil refineries and a number of major ports.
ExxonMobil said on Sunday it had closed its massive Baytown refining complex - the second-largest in the country.
US authorities said about 22% of crude production in the Gulf of Mexico, accounting for more than 375 000 barrels a day, was shut down.