Ten tornadoes tore through the US state of Texas overnight, killing at least six people and injuring dozens as they leveled homes and downed power lines, local authorities said Thursday.

As day broke, first responders were still searching for survivors in the north of the state, in particular in the small town of Granbury in Hood County, Sergeant Nathan Stringer, a North Texas public information officer, told AFP.

"There were an estimated 10 tornadoes which occurred over North Texas last night," Stringer said in an email. "It is still unknown how many total communities were affected however, there was a large portion of Hood County and the City of Granbury affected."

About 53 people were injured and roughly seven are still unaccounted for, he added.

Hood County Sheriff Roger Deeds stressed that finding survivors was a top priority.

"The main concern is life safety and finding victims that still need our help," Deeds told reporters.

The Rancho Brazos subdivision was especially hard hit and it was there that all the deaths occurred, he said.

"That whole subdivision was affected. All the streets out there."

Pastor Dean Porter of the Lake Granbury Christian Temple watched one of the twisters take shape.

A "funnel cloud began to form, and ... the debris was being pulled up into a rotation," he told CNN. "Power lines were pulled down into the road. It was amazing. It was devastating."

The Dallas Morning News ran photos overnight of swirling dark blue masses in the sky, homes damaged by fallen trees and an 18-wheel truck tipped over onto a compact car.

As the sun came up Thursday, aerial video showed scenes of devastation, including demolished houses.

Hood County is southwest of Dallas.

The National Weather Service warned that more tornadoes were possible Thursday afternoon and evening across parts of the United States, including northeastern Texas.