A seven-year-old Texas boy has died after his father's girlfriend allegedly opened fire as the man drove away with his son and two other children, police said.

At least 11 people died and dozens were injured in tornadoes that swept through the Dallas area and caused substantial damage while six people have died in flooding in the US mid-west.

Tornadoes have swept through the Dallas area in Texas, leaving substantial damage and at least eight people dead, in the latest in a succession of freakish winter weather events across the US.

Three people were killed and at least six injured when tornadoes moved through north Texas.