Several neighborhoods and suburbs of Houston were evacuated Sunday due to flooding brought by tropical storm Harvey.

Mandatory evacuation orders were in place for the town of Rosenberg just outside Houston, with police saying it was unclear if the "bridge into the area will hold as water levels rise. Bridge is only way out".

Harvey first made landfall as a hurricane on Friday night in Rockport, Texas, about 220 miles (354 kilometers) southwest of Houston, bringing maximum sustained winds of 133 mph (215 kph).

Several flash flood and tornado warnings have been issued for the region.

The National Weather Service issued a "surviving catastrophic flooding" advisory, reminding people of the dangers of getting trapped in their attics: "Get on the roof and call 911".

The NWS said the storm was slowly heading back into the Gulf of Mexico on Sunday, dropping heavy rainfall over the area on Sunday night and into Monday morning.

As of Sunday evening, parts of the Houston metropolitan area were reported to have received over 25 inches (63.5 centimetres) of rain, with authorities forecasting up to 50 inches throughout the week.

The NWS added that the flooding was "catastrophic, unprecedented, and life threatening" and would continue into the week.

The US coast guard said that it had made 1,200 rescues so far, including 200 from the air using helicopters.

The Houston school district has closed all campuses and offices for the rest of the week, and will not reopen until Sep. 5.

Harvey is the most powerful storm to hit the United States mainland in 12 years.

The storm has killed at least five people and injured dozens, the NWS said Sunday.

The storm closed Houston's two most important airports - the William P. Hobby Airport and the George Bush Intercontinental Airport.

US President Donald Trump said Sunday that he would visit the affected areas as soon as possible.

"Floods are unprecedented, and more rain coming. Spirit of the people is incredible", he said.