An explosive device equivalent to 7 kg of TNT caused Friday evening's deadly derailment of a train travelling from Moscow to St. Petersburg, the Russian federal security chief said on Saturday.

At least 26 people were killed when three carriages of the Nevsky Express came off the rails near the town of Bologoye in the Tver Region, approximately halfway bewteen the capital and St. Peterrsburg.

The death toll is expected to rise, with unconfirmed reports of up to 39 deaths.

"Preliminary information indicates that an improvised device equivalent to 7 kg of TNT exploded," Alexander Bortnikov said. Prosecutors have opened a criminal case on charges of terrorism.

The luxury train, which can travel at 200 kilometers per hour (124 mph), was carrying some 650 passengers and 29 railroad employees.

Russian President Dmitry Medvedev has instructed Emergency Situations Minister Sergei Shoigu to take all necessary measures to extend assistance to those affected. He also instructed the Interior Ministry to "keep the situation under control," adding that the "situation is tense as it is."

A similar derailment, also caused by a blast, occurred on the same route in August 2007, injuring 60 people.

The blast has raised fears of a resurge of terrorist attacks in the Russian capital and other major cities. Russia was hit hard by terrorism in the 1990s and the early years of this decade, but there had not been a major incident outside the volatile North Caucasus region since 2004.

MOSCOW, November 28 (RIA Novosti)