A 15-year-old girl and a truck driver have died after a bridge collapsed into a river in south-western France.

The 150-metre-long suspension bridge in Mirepoix-sur-Tarn, near Toulouse, collapsed about 8am (local time) on Monday as a truck, a car and possibly a van were crossing, the local prosecutor said.

The bridge was limited to 19-tonne vehicles.

Television images showed one car nose-down in the water, with only its rear lights and bumper above the surface. The mother of the dead teenager was among three people rescued from the water, the prosecutor said.

Five people were also seriously injured in the collapse, including two rescuers and locals who had tried to save victims.

Witnesses told the BBC that a school bus crossed less than a minute before the structure collapsed.

Hours after the teenager was declared dead, Eric Oget, mayor of Mirepoix-sur-Tarn, said on French TV station BFM TV the truck driver had also died.

The local prefect Etienne Guyot said earlier the full number of casualties was not yet known. Early reports of there also being a van on the bridge when it collapsed are now thought to be wrong.

The bridge was inspected every six years and had shown no sign of structural weakness at its last inspection in 2017, the Haute-Garonne regional council told Reuters.

A criminal investigation into the causes of the accident has been opened.

French politicians last year called for more money to check and repair bridges across the country after the collapse of a motorway viaduct in Genoa, Italy, killed 43 people. In June, a government report found 25,000 French bridges needed urgent repair.

-with AAP