Thunderstorms have killed four people and knocked out power to nearly a million homes and businesses in south-east Texas.

"We have a storm with 100 mph winds, the equivalent of Hurricane Ike, considerable damage downtown," said the mayor of Houston, John Whitmire.

Two deaths came from fallen trees and one from a crane blown over.

The storm has now moved on to neighbouring Louisiana with flood warnings in place for the Gulf Coast.

In Houston, traffic lights were out, office windows blown through and glass strewn across the city's streets.

Residents were warned to stay inside by the mayor, speaking on Thursday evening.

"Stay at home tonight. Do not go to work tomorrow, unless you're an essential worker.

"Stay home, take care of your children. Our first responders will be working around the clock."

Flash flood and severe thunderstorm warnings were issued for multiple counties heading into the evening, according to Houston's National Weather Service office.

The city was working through a backlog of emergency calls, most of them about gas leaks and downed wires, the city's fire chief said.

Nearly one million customers were without power in Texas as of late Thursday night, according to utility tracker PowerOutage.us.

The vast majority of outages are in Harris County, which contains Houston and is home to more than 4.7 million people.

In Louisiana there are 215,000 households without power.

More than 30 million people are at risk of severe weather on Friday across parts of the Gulf Coast.

Last month, another deadly storm swept through the city when tornado Katy killed one and injured 10.