Security has been tightened across Russia...

Aside from Chechnya, violence is also a regular occurrence in the neighboring republics of Ingushetia and Dagestan.

If terrorism is confirmed as the cause of the blasts, this will be the first major terrorist incident in the Russian capital since the autumn of 2004, when ten people were killed in a suicide bomb attack outside a north Moscow metro station.

The explosion was part of a series of terrorist attacks that also saw 90 people die in two plane bombings and the deaths of over 300 people, many of them children, when Chechen terrorists seized a school in Beslan.

A bomb also hit a Moscow-St. Petersburg express train last November, killing 27 people.

MOSCOW, March 29 (RIA Novosti)