Russian officials investigating the Friday train crash that killed at least 26 people, have confirmed that an improvised bomb caused the fatal derailment.

The head of Russia’s counter-terrorism services announced that the bomb, hidden on the railway line between Moscow and St Petersburg, contained about 7 kg of TNT explosive.

Two large cranes were deployed on Sunday to move wreckage, as rescue workers continued to search for 18 missing passengers. Almost 100 people were being treated in hospital, many with severe injuries.

The elite Nevsky Express was carrying 682 passengers and 29 crew from Moscow to Russia's second city St Petersburg. It was derailed at 9 30 pm on Friday, close to the village of Uglovka, 250 miles north west of Moscow.

Witnesses from among the passengers have described the crash.

One survivor told Russian radio station Ekho Moskvy; “We heard an almighty smack. It seemed to me as if we had lost a wheel or smashed through some kind of obstacle. I didn't hear any explosion.”

Many passengers were unaware that the train had broken up, with the last three wagons of the 14-carriage train flying off the rails.

"The first wagon was 1,5-2 kms away from the rest of the train. The second had completely flipped over. The third had come off the rails, but was near the main part of the train and was still standing vertically. As far as I know nobody from this wagon was seriously hurt," the witness said.

Among the declared dead so far were several senior Kremlin bureaucrats, including the head of Russia's Federal Reserve Agency, and a deputy head in the Federal Fishing Agency. A former St Petersburg senator also died.

There seems little doubt that the Kremlin will point the finger of blame at Islamist insurgents currently waging a guerrilla campaign across the north Caucasus. Rebel fighters have carried out numerous attacks in recent months, including suicide bombings.

"As far as theories go… our main version is that this was an explosion of an unknown device, by unknown individuals. Put simply, it was an act of terror," Vladimir Yakunin, Russia's Railways Minister, said yesterday.

Yakunin said the incident was "analogous" to another derailment on the same line three years ago, also involving the Nevsky Express, in which 19 people were injured. Russian prosecutors blamed that derailment directly on Chechen rebels.