At least 43 people, including a young child, have been killed after a bus and lorry burst into flames after a head-on crash in France.

Motorists desperately tried to smash windows of the burning bus to free passengers near Bordeaux, south-western France.

Most of the victims were French pensioners going on a day trip to walk in the wine producing region of Bearn.

Among the dead were the wood lorry driver and his three-year-old son, sources in France reported.

The crash happened at around 7.30am in the Gironde area about 60km from Bordeaux in the town of Puisseguin.

Witnesses said the crash happened on a dangerous bend and described the aftermath as "like a war zone".

An image released by BFM television showed the carcass of the bus - nothing but a collapsing, charred frame engulfed by smoke.

Helicopters were evacuating severely burned victims, and scores of emergency workers were at the scene.

Government official Pierre Dartout spoke to local media at the scene.

It is the most deadly road accident in France since 53 people, mostly children, died in a bus crash in Burgundy in July 1982.

The bus was about 20 minutes into its trip and the impact was so fierce that both vehicles burst into flames, trapping many inside.

Eight people escaped from the bus after the driver somehow opened the door.

The group were from the village's older people's club. Speaking to RTL radio, this lady says she is a member - but was not on the trip.

Puisseguin Mayor Xavier Sublett said that the truck driver lost control.

The bus driver "tried to avoid it, but the truck came and hit it, and he couldn't do anything except activate the mechanism to open the doors to allow some people to get out," Mr Sublett said.

Other authorities remained cautious about the circumstances of the crash.

An eyewitness who was at the scene soon after said: "I saw a huge, very large, and very intense plume of smoke".

"It was astonishing. Then I saw a police car and, straight after, fire engines everywhere".

"It seemed to me like a war zone and that's not saying it enough. Then, I moved forward on the road and I could tell it was something serious".

Of the eight survivors - three are seriously hurt, while the other five have minor injuries. The bus driver was among them.

The town has been closed off while bodies are removed.

The mayor of nearby Petit-Palais, Patricia Raichin, is one of those directly affected by the tragedy - she has lost three members of her family.

The president of the hiking club in Petit-Palais, where many of the passengers lived, said the group was planning a walking trip.

"On the trip there were people from the pensioners' club in Petit-Palais and from the pensioners' club in Saint-Sauveur," François Decauze told France Info radio.

President Francois Hollande, who is in Greece, said the French government was "totally moved by this terrible tragedy".

Prime Minister Manuel Valls and other senior government ministers are on their way to the site of the crash.

It is France's worst road accident since 1982 when 53 people, including 44 children, died in Beaune in Cote-d'Or.