The remnants of Tropical Storm Harvey has drenched northern Louisiana, leaving behind record flooding that paralysed the US energy hub of Houston, killed at least 35 people and drove tens of thousands from their homes.
Two explosions were reported at a flood-hit chemical plant in Crosby, Texas, 48km northeast of Houston, with one sheriff's deputy sent to the hospital after inhaling toxic chemicals.
The Arkema SA plant had lost power as a result of the storm, causing the organic peroxides stored on site to warm to combustible levels. The company urged people to stay away from the area, warning that further blasts were likely.
The death toll was rising as bodies were found in receding waters. Some 32,000 people were forced into shelters around the region since the storm came ashore on Friday near Rockport, Texas, as the most powerful hurricane to hit the state in a half-century.
By Thursday, the storm was downgraded to a tropical depression. Maximum sustained winds were 48km/h, when Harvey was located about 24km south of Monroe, Louisiana.
The storm's rains wrought the most damage along the Gulf Coast, and the National Weather Service warned that as much as 25.4cm could fall in parts of Mississippi, Tennessee and Kentucky.
The Houston Fire Department will begin a block-by-block effort on Thursday to rescue stranded survivors and recover bodies, Assistant Fire Chief Richard Mann told reporters.
Nearly 76.2cm of rain hit the Port Arthur, Texas, area, the National Weather Service said.
"Our whole city is underwater," said Port Arthur Mayor Derrick Foreman in a social media post where he also broadcast live video of floodwaters filling his home in the city of 55,000 people, about 160km east of Houston.
Beaumont, near Port Arthur, said it had lost its water supply due to flood damage to its main pumping station. Residents in the city of about 120,000 people would lose water pressure from Thursday morning.
Fort Bend County ordered a mandatory evacuation on Thursday for areas near the Barker Reservoir, which was threatening to flood. The reservoir is about 32km west of Houston.
The county did not say how many people the evacuation order would affect.
Clear skies in Houston on Wednesday brought relief to the energy hub and fourth-largest US city after five days of catastrophic downpours. The first flight out of Houston since the storm hit boarded on Wednesday evening.
Mayor Sylvester Turner said he hoped the port of Houston, one of the nation's busiest, would reopen soon.
Police in Houston's Harris County said 17 people remained missing.
Nearly 200,000 homes and businesses in Louisiana and Texas were without power on Thursday, utilities reported.
Flooding shut the nation's largest oil refinery in Port Arthur in the latest hit to US energy infrastructure that has sent petrol prices climbing and disrupted global fuel supplies.
At least $US23 billion ($A29 billion) worth of property has been affected by flooding from Harvey just in parts of Texas' Harris and Galveston counties, a Reuters analysis of satellite imagery and property data showed.
The area affected is larger than that hit by 2005's Hurricane Katrina, which killed more than 1,800 people in New Orleans, and 2012's Superstorm Sandy, which killed 132 around New York and New Jersey, he said.