Four people died in Texas on Thursday as heavy storms with winds up to 100 miles (160 kilometers) per hour lashed the southwestern US state, local authorities said.

Streets in downtown Houston were covered in glass from blown-out windows, video and photos posted to social media showed, as the National Weather Service warned of "severe" thunderstorms and possible tornadoes.

Downed trees and power lines littered residential areas, and around a million customers in the state had lost power -- including more than 800,000 in Harris County, where Houston is located.

The mayor of the city, the largest in Texas, said four people had died.

"Mostly from fallen trees, people trapped in their cars," Mayor John Whitmire told reporters, though exact details were not available.

Winds had reached speeds of between 80 and 100 miles per hour, he said.

Whitmire urged residents to avoid downtown, adding that public schools would be closed on Friday and asking non-essential workers to stay home as well.

Restoring power "will take us into late tomorrow," he said.

The mayor and the city's office of emergency management (OEM) were working with first responders to assess the damage, the city said.

Residents were asked to stay off the roads and avoid unnecessary travel, according to the OEM.

With about 2.3 million people, Houston is the fourth most populous city in the United States, with an economy driven in large part by the oil and petrochemical industry.