At least 39 people have been killed in a train crash that Russian authorities are treating as an act of terrorism.

Prosecutors in Moscow said they suspected that a bomb blast was responsible for the derailment of a passenger train on the main line between the capital and St Petersburg.

Reports said a crater had been seen next to the tracks, though other sources at the scene denied this was the case.

The Nevsky Express, one of Russia’s busiest trains, was carrying 661 passengers when it came off the rails at 2134 local time (1834 GMT) about 350 miles north of Moscow.

Rescuers have so far found 39 bodies and almost 100 casualties, with at least 18 people unaccounted for.

Russian Railways chief Vladimir Yakunin told reporters at the scene: “There is objective evidence that ... a blast from an explosive device is one of the explanations for the Nevsky Express incident.”

So far, however, no organisations are known to have claimed responsibility for the accident, which is Russia’s worst train disaster in years.

A terrorist bomb in 2007 derailed the same train near the same location, injuring 30 people. Two residents of Ingushetia, a territory bordering Chechnya, were arrested and charged with carrying out the attack.

Russian prosecutors said they believed ex-soldier Pavel Kosolapov, a former associate of the late Chechen rebel leader Shamil Basayev, was the mastermind behind that blast. Kosolapov is still on the run.