In this photo provided by France Bleu, rescue workers help after a school bus and a regional train collided in the village of Millas, southern France, Thursday, Dec. 14, 2017. A school bus and a regional train collided in southern France on Thursday, killing four children and critically injuring several other people on the bus, the French interior ministry said. (Matthieu Ferri/France Bleu via AP)

Paris - Four children were killed and seven seriously injured after a train smashed into a school bus, splitting it in two, outside the town of Perpignan in southwestern France on Thursday, the interior ministry said.

Television images showed a long line of ambulance and emergency service vehicles near the crossing where the collision occurred, with the bus cut in half, highlighting the ferocity of the impact.

"All my thoughts to the victims and their families of this terrible school bus accident," French President Emmanuel Macron said on Twitter.

Prime Minister Edouard Philippe said he was travelling to the scene, more than 850 km (530 miles) from Paris.

"All emergency services have been mobilised and a crisis coordination unit set up," an official at the local Millas townhall told Reuters.

The interior ministry said seven people in the bus, which was carrying students aged 13 to 17, were seriously injured among the 16 aboard.

The reasons for the crash are not yet known, officials said.

A spokeswoman from state-owned rail operator SNCF said the train was running at 80 kilometres an hour at the time of the accident and 25 people were on board. Three of those were slightly injured.

"It was a classic crossing, well equipped and lit. Several witnesses confirmed that the barrier had come down so it worked," the spokeswoman said, adding that the investigation would have to clarify what had happened.

France has suffered several train incidents in the past few decades. In 2013, a train derailment in central France killed at least six people. One of the deadliest was in 1988, when a commuter train heading into Paris’ Gare de Lyon crashed into a stationary train, killing 56 people, after its brakes failed.