Two young sisters have been rescued after clinging to a tree for four days to escape torrential floods which swept away their parents and a brother, Mexican authorities said Sunday.

Rescue workers were still searching for the oldest sister and another brother of Lisbeth, 10, and Lesli, nine, who remain missing after their car was swept from a bridge between Matamoros and Ciudad Victoria, in the northeastern Tamaulipas region.

The two girls were rescued on Thursday after clinging to the branches of a tree for four days, said local official Marisela Cantu told AFP from the area bordering the US state of Texas.

They were dehydrated and covered in insect bites but otherwise in good health after they were spotted by fishermen on the banks of the river, swollen after Tropical Storm Alex swept over northeastern Mexico.

The bodies of their parents and 13-year-old brother have been found, but rescue teams were Sunday still searching for their older sister, 15, and another brother who were also in the car.

Alex brought a trail of damage to northeastern Mexico, killing 15 people and leaving 40,000 homeless.

On Saturday, firefighters rescued two Hondurans who were swept away as they tried to sneak across the Rio Bravo which marks the border between Mexico and the United States, local official Ernesto Rivera Gomez said.