US President Donald Trump warned Monday that Texas faces a "long and difficult road" to recovery after floods triggered by an unprecedented rainstorm.
"There's probably never been anything like this," Trump said of Tropical Storm Harvey, which hit the Gulf Coast at the weekend and inundated the Houston region.
"It's the biggest ever, they are saying it is the biggest, it's historic. It's really like Texas when you think about it," Trump said.
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 recognize there are many other people out there in distress situations and we intend to get to every one of them."
Dams opened
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" nine trillion gallons of 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.
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 six 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 neighboring Louisiana, freeing up federal funds.
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.
Roads completely submerged
"It's crazy to see the roads you're driving on every day just completely under water," Houston resident John Travis told AFP.
While the disaster response is now focused on the huge city, center 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 housewife 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 nothing 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.
'Life and safety'
"The focus must be on life and safety," Trump wrote in a series of tweets about the disaster, his first major domestic challenge since taking office in January.
At least three deaths have been blamed on Harvey, which has spawned tornadoes and lashed east and central Texas with torrential rains.
"The breadth and intensity of this rainfall are beyond anything experienced before," the National Weather Service said.
The NWS said that between June 1 and Sunday, Houston received 46 inches (117 centimeters) of rain. "Annual average rainfall is 49.77... Almost a yr of rain in 3 months," it wrote on Twitter.
Houston opened community centers to shelter people forced out of their homes.
"Even if there's a lull today, don't assume the storm is over," Turner said.
Source: Houston Chronicle / AAP
Beyond anything experienced
The National Weather Service said that between June 1 and Sunday, Houston had received 46 inches (117 centimeters) 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.
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."
'Landmark' disaster
The Federal Emergency Management Agency (FEMA) said there should be no illusions about the long-term impact.
"This disaster will be a landmark event," FEMA director Brock Long told CNN, adding 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 Sunday it had closed its massive Baytown refining complex -- the second-largest in the country.
US authorities said about 22 percent of crude production in the Gulf of Mexico, accounting for more than 375,000 barrels a day, was shut down.
But Abbott said the oil industry was well prepared.
"They have the ability to ratchet up back up there quickly," he said on Fox News Sunday, predicting a "one- or two-week downturn."
US neighbors Mexico and Canada offered solidarity and aid.
Mexico's foreign ministry said it offered help to deal with Harvey, "as good neighbors should always do in trying times."
Prime Minister Justin Trudeau tweeted that Canadians "are keeping the people of Texas in our thoughts -- we're ready to offer any assistance needed to help recover from this disaster."
