HOUSTON — Crews went door-to-door Sunday in parts of western Houston, shutting off power and warning anyone still in homes left soggy by Harvey that more flooding was possible as engineers relieved pressure on overtaxed city reservoirs.

Some people took a break from their cleanup efforts in the sweltering heat to worship on a declared National Day of Prayer, while others worried about looters and scavengers in storm-ravaged neighborhoods.

Houston officials stressed that the recovery was already starting the renewed flood threat, but an official in the town of Liberty, northeast of the city, said some people in outlying areas there had yet to even return to their homes.

"This will last for some people for months, if not years," said Liberty fire Chief Brian Hurst.

Authorities also warned that fires from a chemical plant rocked by previous blazes and explosions were still a possibility in the suburb of Crosby and residents of nearby Beaumont remained without clean drinking water.

Authorities started a controlled burn Sunday of the highly unstable compounds at the chemical plant that flooded because of Harvey. The Harris County Fire Marshal's Office said in a statement Sunday the decision was made to take "proactive measures" to ignite the remaining trailers at the Arkema plant. The office said that it doesn't pose any additional risk to the public. The 1.5-mile evacuation zone around the plant is still in place.

At least 4,700 Houston dwellings were under new, mandatory evacuation orders, though about 300 people were thought to be refusing to leave.

Mayor Sylvester Turner said anyone staying in already-waterlogged homes was endangering themselves and first responders. Harvey hit Texas Aug. 25 as a Category 4 hurricane, but brought the worst flooding to Houston and other communities as a tropical storm. It is blamed for at least 44 deaths.

The Environmental Protection Agency said Sunday that more than 200 public water systems were either shut down or under boil-water notices, and Gov. Greg Abbott said that he believed Harvey's toll would exceed that of Hurricane Katrina, which devastated Louisiana and Mississippi 12 years ago.

Still, Turner insisted that much of the nation's fourth-largest city was hoping to get back on track by Tuesday. "The city of Houston is open for business. Anyone who was planning on a conference or a convention or a sporting event or a concert coming to this city, you can still come," Turner said on CBS's Face the Nation.

Meanwhile, a debate over federal aid for Hurricane Harvey intensified in Washington.

Treasury Secretary Steve Mnuchin said that relief funds for repairing the devastation left in Texas should be tied to a bill that would raise the federal debt ceiling.

"Our first priority is to make sure that the state gets money — it is critical," Mnuchin said in an interview on Fox News Sunday. "And to do that, we need to make sure we raise the debt limit."

Information from the Associated Press and the New York Times was used in this report.