At least 18 people have died and 13,000 more have been rescued as Tropical Storm Harvey dumps record rainfall in and around Houston.
As thousands of people fled their deluged homes, Houston’s mayor, Sylvester Turner, imposed a curfew amid reports of looting and armed robberies.
Officials say they have received disturbing reports of people impersonating Homeland Security special agents and telling residents to evacuate in order to rob their homes.
The heaviest tropical downpour in US history has left an area 15 times the size of Manhattan under water, according to the Houston Chronicle newspaper.
City officials readied temporary housing for about 19,000 people, with thousands more expected to flee as flooding entered its fourth day and space ran out in cramped shelters.
With many reported missing, the number of dead is expected to rise once floodwaters start to recede.
The storm was set to make landfall again later near the Texas-Louisiana border.
It was expected to produce an additional 15 cm to 30cm (6in to 12in) of rain to the north and east of Houston as it moves into southwestern Louisiana.
It is projected to weaken as it moves inland to the northeast and be over Mississippi by Thursday, the National Weather Service said.
President Donald Trump visited the affected area on Tuesday, telling rescue organisers: “This was of epic proportion, nobody has ever seen anything like this.”
The President paid a visit to Corpus Christi, where the storm made landfall, and will not travel to Houston.
On Tuesday, residents within 1.5m (2.4km) of a chemical plant in Crosby were ordered to evacuate due to the rising risk of an explosion and subsequent leak.
A breached levee south of Houston and two overflowing reservoir dams outside the city are also adding to the rising water levels.
The National Hurricane Centre warned of “ongoing catastrophic and life-threatening flooding” continuing across south eastern Texas.
And The National Weather Service warned of dangerous flash flooding pushing northeast over the coming days.
Rescue services say that make-shift shelters are beginning to run out of space, as two and possibly three “mega-shelters” are being planned to accommodate evacuees in Houston.
With rescue services overstretched, civilians with boats and trucks are responding to calls for help and helping saving lives.
On Wednesday, Singapore’s defence ministry sent four military helicopters to assist with the relief effort, airlifting troops, evacuees and supplies.
Damage is expected to run into tens of billions of dollars, making it one of America’s most expensive natural disasters.