An explosion hits the Moscow metro during Friday morning rush hour, killing at least 40 people, injuring more than 100, and sending clouds of smoke through the tunnel, CNN reported.

A severe fire broke out in the underground train and passengers were being evacuated from Avtozavodskaya station, said Viktor Beltsov, spokesman for the Emergency Situations Ministry.

Beltsov said he could not confirm the cause. He said that rescuers were already on the site, and numerous ambulances could be seen outside the station entrance.

Moscow hospitals were put on alert. Interfax, citing emergency officials, said that the explosion occurred in the second wagon of a train after it left the Paveletskaya station near the center of the Russian capital and headed southeast to Avtozavodskaya station.

The train wagon was badly damaged, the Interfax and ITAR-Tass news agencies reported.

Police immediately barricade the two metro stations and stopped all traffic on the metro, clogging up the capital's streets.

Russian prosecutors said they could not rule out terrorism, but that it was too early to say definitively.

Interfax news agency quoted a police source as saying that first interviews with evacuated passengers led to the conclusion that the blast may have been a "terrorist act."

Russian President Vladimir Putin was immediately informed. The Russian capital has been on alert for terrorist attacks following a series of suicide bombings that officials have blamed on Chechen rebels.

In December, a female suicide bomber blew herself up outside the National Hotel across from Moscow's Red Square on Tuesday, killing at least five others.

Two suicide bombers blew themselves up at a Moscow rock concert in July, killing themselves and 14 other people. That was followed five days later by an aborted suicide bomb attack at a central Moscow restaurant that killed the sapper trying to defuse the bomb.

The suicide bomber was arrested and is currently awaiting trial.

In August 2000, a bomb exploded at a crowded pedestrian underpass filled with kiosks at Pushkin Square, a popular meeting place located near a metro line.

The attack was initially blamed on Chechen rebels, but some police later said that a turf battle between rival businessmen or criminal gangs could have been the motive.

(China Daily February 6, 2004)