BRETIGNY-SUR-ORGE, France (AFP) - At least six people were killed and many injured on Friday after a speeding train derailed and hit the platform at a station just south of Paris, in France's worst rail accident in 25 years.

Witnesses said the site of the crash resembled "a war zone", with one describing walking over a decapitated body to exit a carriage that had been thrown on its side.

Rescuers were searching late on Friday for passengers who may still be trapped inside overturned carriages.

The train, a regional service heading from Paris to the west-central city of Limoges, hurled off the line as it passed at high speed through the station at Bretigny-sur-Orge, about 25km south of Paris.

The interior ministry said in an updated toll that at least six people had been killed, nine seriously injured and taken to hospital by helicopter and 17 lightly injured.

Interior Minister Manuel Valls had earlier spoken of seven dead and "dozens injured", warning that the death toll was likely to rise.

Some 300 firefighters, 20 paramedic teams and eight helicopters were deployed to the area. All Paris region hospitals were put on alert to deal with the injured.

In total, 192 people were treated by emergency services, officials said.

The head of the SNCF national railway, Mr Guillaume Pepy, told reporters at the scene that six carriages had derailed during the accident. The train's third and fourth carriages derailed first and the others followed, he said.

A clearly emotional Pepy expressed the rail company's "solidarity with the victims and their families".

"Rail catastrophes are something that upset everyone and all of those who are committed to our national rail service," he said.

"We don't yet know the reasons for this derailment," Mr Pepy said, adding that investigations would be carried out by the rail service, judicial authorities and France's BEA safety agency.

After travelling to the scene and meeting with officials, President Francois Hollande confirmed that three investigations had been launched "to determine what happened at this station".

"We should avoid unnecessary speculation, what happened will eventually be known and the proper conclusions will be drawn," Mr Hollande said.

Passenger Marc Cheutin, 57, told Agence France-Presse (AFP) he had to "step over a decapitated person" after the accident to exit the carriage he had been travelling in.

"Shortly after departure, just as I was getting into my book, we felt a first shock that shook the carriage I was in... Then there was a second shock and the carriage lifted up, then a third and a fourth and the carriage went over on its side," he said.

The prefect's office said a "red alert" plan had been activated following the accident, which officials said happened at 5.14pm (11.14pm Singapore time), minutes after the intercity train left the Paris-Austerlitz station.

"The train arrived at the station at high speed. It split in two for an unknown reason. Part of the train continued to roll while the other was left on its side on the platform," a police source told AFP.

"It was not a collision and it was not a problem with the speed," a source with the SNCF told AFP.

The train normally travels through the station at a speed of about 150km per hour, officials said.

A witness who had been waiting for a train at the station, Mr Vianey Kalisa, told AFP: "I saw a lot of wounded people, women and children trapped inside (the carriages).

"I was shaking like a child. People were screaming. One man's face was covered in blood. It was a like a war zone."

Bretigny Mayor Bernard Decaux told newspaper Le Parisien there was chaos after the accident.

"Everyone is running in every direction, there is panic," he said. "It is an apocalyptic scene. We are trying to organise things."

The accident occurred as many in France were departing for the start of their summer holidays ahead of Bastille Day on Sunday.

In Brussels, EU Commission chief Jose Manuel Barroso expressed his condolences.

"The derailment of the train casts a pall over the holiday departures as your country is preparing its national day," he said in a message to Mr Hollande.

The derailment was France's worst rail accident since an SNCF commuter train crashed into a stationary train at Paris's Gare de Lyon terminal in 1988, leaving 56 dead.