Tropical Storm Harvey has triggered a fire and two explosions in a flood-hit chemical plant in Crosby, Texas as the remnants of the storm moves towards the Louisiana border.

Nearly a week after Hurricane Harvey’s relentless rain and flooding hit Houston, storm-related power outages prompted two explosions at a Arkema SA chemical plant, with one sheriff’s deputy sent to the hospital after inhaling toxic chemicals.

Company officials described the smokes as a “noxious” irritant, which formed after refrigeration systems on a truck used to store the chemicals failed, causing them to overheat on Thursday.

“The plume is incredibly dangerous,” Federal Emergency Management Agency (FEMA) administrator Brock Long said at a news briefing.

The area within 2.4km radius was evacuated after nine to 12-metre flames and black smoke was reportedly seen at the plant.

But according US Environmental Protection Agency (EPA) he amount of toxic materials released appeared to be too small to present a public health concern.

More than 20 first responders were treated at the scene for chemical exposure, with fire authorities reporting some suffered irritated eyes from the smoke, The New York Times reported.

It comes as the latest in a string of tragic incidents in the Houston area, which has seen at least 35 people die as a result of the storm.

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.

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.

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.

Trump to donate $1 million to Harvey relief

US President Donald Trump walks through the the Texas Department of Public Safety Emergency Operations Center in Austin, Texas. Photo: Getty

The White House says President Trump is pledging $US1 million ($A1.3 million) in personal funds to Harvey storm relief efforts.

White House Press Secretary Sarah Huckabee Sanders made the announcement at a briefing Thursday.

She said Trump’s calling on reporters to help decide which specific organisation he will give to.

Trump has been criticised in the past for giving far less of his income to charitable causes than many other multi-billionaires.

– with AAP