A least 16 people have been killed in Texas and Oklahoma after heavy storms and flooding swept through the two states.

"A lot of folks drove their car into high water and had to abandon those vehicles," Houston Mayor Annise Parker said at a news conference.

Two of the dead in Houston were found in their cars and another two were found in a bayou, Reuters reported.

President Barack Obama said on 27 May that he had assured Texas Governor Greg Abbott, who declared a state of disaster in 40 counties, that the federal government would help the state recover from the floods.

The governor added that he had deployed the state's National Guard and was worried the death toll could rise. "It's devastating to see what I saw on the Blanco River when this tidal wave of water just swept away neighbourhoods," he said, recalling a disaster area in central Texas.

Houston resident Dutch Small, 40, who climbed on to the roof of his car when the water came up to his knees inside his vehicle, told Reuters: "It happened so fast. Every person that died in the flooding, I know what was going through their minds. They didn't measure the threat accurately. They were like me."

The National Weather Service issued tornado and thunderstorm watches and said more rain is expected this week in Texas and Oklahoma.