(Updates death toll, adds details)

PARIS, June 2 (Reuters) - Seven children were killed when a train collided with a school bus at a level crossing in eastern France on Monday, French authorities said.

Three people were seriously hurt and 24 suffered less serious injuries, police said.

Fifty pupils from a school in the small town of Margencel, five parents and the driver were on board the bus, which was taking the children on a school trip to a local historical site.

The children were believed to be 12 or 13 years old.

The accident occurred just before 2:00 p.m. (1200 GMT) when a regional train struck the bus at a level crossing at Mesinges, near the town of Allinges in the mountainous Haute Savoie area near the border with Switzerland.

National train operator SNCF and rail network operator RFF said they would launch investigations to determine the causes.

RFF said early indications were that the level crossing, which was not considered high risk, was functioning normally. RFF said it was the most serious accident at a level crossing in about 30 years.

A woman in a car behind the bus told France Info radio she saw the bus start to cross the track when the red warning lights were already flashing.

"The train came and it cut the bus in two," said the weeping woman, whose name was not given on the radio report.

The drivers of the bus and train both survived.

The Education Ministry said counsellors were on hand to help pupils from the school cope with the trauma.

Several ministers were on their way to the site and President Nicolas Sarkozy expressed shock and sympathy. (Reporting by Laure Bretton, Christian Hartmann and Thierry Leveque; writing by Estelle Shirbon; editing by Andrew Dobbie)

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