Emergency crews raced to pull people from cars and homes as flood waters rose across southeast Texas, rescuing more than 1,000 people in the Houston area as Tropical Storm Harvey pounded the region.
The most powerful storm to hit Texas in more than 50 years has killed at least two people.
The death toll is expected to rise as Harvey lingers for days above the US state, triggering more floods, storm surges and tornadoes.
US President Donald Trump will travel to Texas on Tuesday to survey the damage caused by Hurricane Harvey.
White House spokeswoman Sarah Sanders said: "The president will travel to Texas on Tuesday. We are coordinating logistics with state and local officials.
"We continue to keep all of those affected in our thoughts and prayers."
Earlier, Texas mayor Sylvester Turner said that floods were being spread "all over". He added that 40 boats will be added for rescuing people from high waters that have filled neighbourhoods and highways.
Harvey is forecast to arc slowly toward Houston through Wednesday.
At least two people have died in Texas following ‘catastrophic’ flooding in the wake of Hurricane Harvey pic.twitter.com/6yKfsdfLG9
— RTÉ News (@rtenews) August 27, 2017
Emergency services in Houston told people to climb onto the roofs of their houses rather than into their attics to escape rapidly rising waters.
Authorities warned the city's more than two million residents not to leave flooded homes because many of the city's roads are underwater.
The Twitter account of the sheriff of Harris County, which includes most of Houston, was inundated with rescue requests.
Emergency services were stretched, and Mr Gonzalez could only tell some people asking for help his teams were doing the best they can.
"All agencies care but everyone simply operating at maximum capacity," he tweeted at one point.
Chris Bohill, originally from Co Down but now living in Houston, said that the heavy flooding is "horrendous".
Speaking to RTÉ News, Mr Bohill said: "I've never seen anything like it in my life, it is sheer devastation.
"Where I live there used to be a park and baseball field, now it's just like an ocean. It's just phenomenal."
Chris Bohill, originally from Co Down but now living in Houston, says the heavy flooding in Houston is 'horrendous' pic.twitter.com/iKZ8rjZte5
— RTÉ News (@rtenews) August 27, 2017
Houston's Bush Intercontinental Airport canceled all commercial flights after flooding inundated access roads, further isolating the city.
The city's only other airport, Hobby International, had earlier stopped all flights "due to standing water in runways."
The Ben Taub Hospital in Houston's Medical Center was evacuated. An American Red Cross emergency shelter was forced to shut due to flooding and the group opened two more, including one in a convention centre in downtown Houston.
STUNNING images of Aldine Westfield at Beltway 8. In the distance, you can see multiple cars (hopefully abandoned) underwater. #KHOU11 pic.twitter.com/7e8taEOIBs
— Brandi Smith (@BrandiKHOU) August 27, 2017
The second confirmed fatality from Tropical Storm Harvey came yesterday evening as an elderly woman attempted to drive through flooded streets on Houston's west side, said Sergeant Colin Howard of the Houston police department.
"It appeared that her vehicle went into high water and she drowned as a result," he said. The victim was not immediately identified.
On Friday night, a man died in a house fire in the town of Rockport, 48km north of the city of Corpus Christi. Another dozen people in the area suffered injuries including broken bones, another official said.
Energy production in the state was disrupted as several refineries and offshore platforms closed down, triggering a rise in gasoline prices.
More than 45% of the country's refining capacity is along the US Gulf Coast, and nearly a fifth of the nation's crude oil is produced offshore in the Gulf of Mexico.
Harvey slammed into Texas late Friday as a Category 4 hurricane with winds of 210km per hour,making it the strongest storm to hit the state since 1961.
The storm ripped off roofs, destroyed buildings, flooded coastal towns and had cut off power to nearly 230,000 people in Texas as of Saturday night.
Texas Governor Greg Abbott said he was activating 1,800 members of the military to help with the statewide cleanup, while 1,000 people would conduct search-and-rescue operations.
Harvey was downgraded to a tropical storm on Saturday because its winds have slowed. But authorities issued stark warnings on the threat posed by days of torrential rain.
