US President Donald Trump has asked Congress to approve $7.85bn in initial emergency funding to deal with the devastation wrought by Hurricane Harvey, as areas hit by the powerful storm moved into a recovery phase.

"The administration believes additional federal resources are necessary to continue to fund critical, and often life-saving, response and recovery missions," Trump's letter, sent to legislators on Friday, stated.

The figure, substantially larger than the around $6bn previously floated, would soon be followed by another request for $6.7bn, according to a letter to House Speaker Paul Ryan cited by media.

READ MORE: Texas' poor hit hardest by floods after Harvey

Harvey, which made landfall as a Category 4 hurricane with record-breaking rainfall, has left tens of thousands homeless and at least 42 people dead, according to authorities in the US state of Texas.

AccuWeather, a private meteorological agency that delivers weather data and forecasts to a large portion of the US media, predicted the cost of Harvey will exceed $190bn, or about one percent of US gross domestic product.

Houston Mayor Sylvester Turner said rescues were still proceeding outside the city, and workers were going door-to-door in the Texas city to make sure no one has been missed, especially seniors or people who are disabled.



"There are still many rescues in the region, but with regards to the city of Houston, we are starting to move into the recovery phase with the number of rescues being dramatically down," Turner said at a briefing.



He also announced a voluntary evacuation for people living near two reservoirs, saying the US Army Corps of Engineers must release water from them for the next 15 days to avoid a larger catastrophe the next time it rains.

Risk of more flooding

Local flood control authorities said nearly for trillion litres of water fell on Harris County, where Houston is located, and 136,000 buildings were flooded, after Harvey made landfall as a Category 4 hurricane one week ago.

Some 70 percent of the county's 4,500sq km have been covered in half a metre of water, said Francisco Sanchez, spokesman for local emergency officials.

In Washington, Trump met religious leaders and declared Sunday a national day of prayer for the victims of Harvey.

Texas is "healing fast thanks to all of the great men & women who have been working so hard" in the wake of massive destruction from the hurricane, he wrote on Twitter, adding that there had been no outbreak of crime.

"There was an outbreak of compassion only," Trump said.

The president is scheduled to travel later on Saturday to the Houston area to view flood damage.

While water levels were dropping in Houston, authorities are still dealing with massive damage and the need to restore basic services.

Meanwhile, another fire has broken out at Houston-area chemical plant that lost power after Harvey, sending black smoke into the air.

Aerial video showed a towering fire and smoke coming from the Arkema plant in Crosby on Friday evening. The fire came after a container of organic peroxides exploded and caught fire early on Thursday, sending acrid smoke into the air.