About 100,000 homes have been affected by Storm Harvey and the flooding it brought to parts of the southern US, a White House aide has said.
Homeland Security adviser Tom Bossert said the White House would ask Congress for emergency funding to help those affected, some of whom are uninsured.
Meanwhile US President Donald Trump is said to have pledged $1m of his own money to the relief effort.
At least 33 people have died since the storm hit Texas last Saturday.
Firefighters have begun a door-to-door search for survivors and bodies in an operation that could take up to two weeks.
Oil refineries and pipelines have been shut down, and a fire at a chemical plant near Houston sent up a plume of smoke.
More fires and possibly explosions are expected at the Arkema liquid organic peroxide plant, and the area around it has been evacuated up to a radius of 1.5 miles (2.4km).
Storm Harvey has now been downgraded to a tropical depression, but heavy rainfall is expected from Louisiana to Kentucky over the next three days and flood warnings remain in effect for south-east Texas and parts of south-west Louisiana.