A tornado has caused damage and possible injuries as it touched down in Texas, as severe storms moved through the state and Oklahoma.

Forecasters issued tornado and severe thunderstorm warnings for parts of both states, after some heat records were broken during Saturday in south Texas and residents were warned temperatures will remain above average over the long weekend.

A tornado crossed into northern Denton County in Texas late on Saturday and overturned a number of trucks, stopping traffic on Interstate 35.

Denton County community relations director Dawn Cobb said the tornado was confirmed near Valley View, moving east at 40mph, prompting the National Weather Service to issue a tornado warning for the area.

The storm damaged homes, overturned motorhomes and knocked down power lines and trees throughout the area. Ms Cobb said the number of injuries in the county is not yet known.

The fire service in the city of Denton said emergency personnel were responding to a marina “for multiple victims, some reported trapped”.

Earlier on Saturday night, the National Weather Service’s office in Norman, Oklahoma, said the tornado warning was for northern Noble and far southern Kay counties, an area located to the north of Oklahoma City. “If you are in the path of this storm take cover now,” it said on social media.

A following post said storms had exited the area but warned of a storm moving across north Texas that could affect portions of south central Oklahoma.

Excessive heat, especially for May, is the danger in south Texas, where the heat index was forecast to approach 49C in some spots during the weekend. Actual temperatures will be lower, but the humidity will make it feel that much hotter.

Sunday looks like the hottest day with record highs for late May forecast for Austin, Brownsville, Dallas and San Antonio, forecasters said.

Brownsville and Harlingen near the Texas-Mexico border already set new records on Saturday for May 25 – 37C and 38C respectively, according to the weather service.

Wildfire warnings are in place across Texas, New Mexico, Oklahoma, Arizona and Colorado.