At least 40 people, mostly elderly tourists starting off on a day trip, were killed Friday when a bus and truck collided on a country road in wine country in southwest France, officials said.

The cause and circumstances of the crash — near the village of Puisseguin, about 50 kilometers east of Bordeaux — were unclear. Scores of emergency workers rushed to the scene.

The death toll may have been so unusually high because the bus caught fire, an official with the national gendarme service said. The official was not authorized to be publicly named.

French President Francois Hollande, on a visit to Greece, said the government was "totally mobilized" to help after what he called a "terrible accident."

The bus was carrying mostly elderly people from nearby towns on a one-day tourist trip to another site in southwest France, and had just left when it collided with the truck, legislator Gilles Savary said on BFM television. He called it one of the deadliest accidents in recent years.

French media reports said some people managed to escape, notably by breaking windows.

The weather in the region was overcast Friday morning but not rainy.