An explosion derailed a Russian train travelling between Moscow and St Petersburg late on Monday, injuring dozens of passengers but killing no one, officials said.

Officials said the cause of the blast was unknown. "As a result of an explosion at 21:38 (1738 GMT) ... several carriages of passenger train No. 166 from Moscow to St Petersburg were derailed," Russian Railways said in a statement.

The statement said the explosion was caused by "outside action", indicating that it may have been the result of foul play and not just a technical fault.

The derailment happened in Novgorod region, about 500 km (310 miles) north of Moscow, near the village of Malaya Vishera.

Viktor Beltsov, a spokesman for Russia's Emergencies Ministry, said 27 people were admitted to hospital to have their injuries treated. "Three are in a serious condition," he said.

Russian news agencies reported that a total of 60 people were hurt, but most did not require hospital treatment.

"The driver said he heard a loud bang," Alexander Pirkov, an advisor to Russian Railways chief Vladimir Yakunin, told the Vesti-24 television news channel.

"There were 215 passengers on board and some have been injured but there are no fatalities according to preliminary information ... Nine carriages came off the rails."

Interfax news agency, citing law enforcement sources, said a crater measuring about 1.5 metres (5 ft) in diameter was found underneath one of the carriages.

The line between Moscow and Russia's second city of St Petersburg is among the country's busiest.

Russian news agencies reported the line had been closed, but alternative routes would be put into operation to keep trains running.

Russia has a history of violent attacks on civilian targets, many of them carried out by groups linked to a separatist insurgency in the southern region of Chechnya.

But the insurgents have launched no major attacks outside Chechnya and neighbouring regions for at least a year after many of them were either killed or arrested by security forces.