Texas is now in the grip of a flood emergency, following Hurricane Harvey.

In Houston - residents have been told to seek refuge on their rooftops, as the water begins to fill the second storey of buildings.

Mayor Sylvester Turner, who's coming under fire for the way the city has handled the crisis, says the size of the storm is unprecedented:

"Quite frankly every neighbourhood, every community received water and are flooded."

1 News US correspondent Rebecca Wright is in Houston and said, "there are thousands who are tonight still trapped in their homes, with rising flood waters and just nowhere to go. Others are trapped in their cars on freeways."

Several people have died, and the death toll could rise as the storm sparks tidal surges and tornadoes.