Power outages could last weeks in parts of Houston after thunderstorms with hurricane-force winds tore through the city in Texas, an official said.

The storms knocked out electricity to nearly one million homes and businesses.

“We are going to have to talk about this disaster in weeks, not days,” Harris County Judge Lina Hidalgo, the county’s top elected official, said at a news conference.

Houston mayor John Whitmire said four, and possibly five, people were killed after storms swept through Harris County, which includes Houston, on Thursday.

Officials warned residents that it would be a slow clean-up and that some should brace to be without electricity for days, if not longer.

“It was fierce, it was intense, it was quick and most Houstonians didn’t have time to get themselves out of harm’s way,” Mr Whitmire said.

He said at least 2,500 traffic lights were out and warned would-be looters that “police are out in force including 50 state troopers sent to the area to prevent looting”.

The widespread destruction brought much of Houston to a standstill as crews raced to restore power and remove uprooted trees and debris.

School districts in the Houston area cancelled classes for more than 400,000 students and government offices were closed. City officials urged people to stay off roads, many of which were flooded or lined with downed power lines and malfunctioning traffic lights.

At least two of the deaths were caused by falling trees and another happened when a crane blew over in strong winds, officials said.

Mr Whitmire said wind speeds reached 100mph “with some twisters”. He said the powerful gusts were reminiscent of 2008’s Hurricane Ike, which pounded the city.

Hundreds of windows were shattered at downtown hotels and office buildings, with glass littering the streets below. Fallen trees, power lines and broken glass have made some areas impassable, the city said.

The storms were not over on Friday. Gulf Coast states could experience scattered, severe thunderstorms with tornadoes, large hail and damaging winds.

Heavy to excessive rainfall is possible for eastern Louisiana into central Alabama, the National Weather Service said. Flood watches and warnings remained on Friday for Houston and areas to the east.

The powerful storms also struck neighbouring Louisiana on Thursday and left more than 215,000 customers without power at their peak.

The Storm Prediction Centre’s website showed a report of a tornado in Convent, Louisiana, about 55 miles from New Orleans, with multiple reports of trees and power poles down.

There were wind gusts of 84mph at Louis Armstrong New Orleans International Airport and 82mph at New Orleans Lakefront Airport, according to Tim Erickson, a meteorologist at the weather service’s office for New Orleans and Baton Rouge.

The office for New Orleans and Baton Rouge issued a flash flood warning through to Saturday.

Flights were briefly grounded at Houston’s two major airports. Sustained winds topping 60mph were recorded at Bush Intercontinental Airport.

Heavy storms slammed into the Houston area during the first week of May, leading to numerous high-water rescues, including some from the rooftops of flooded homes.