Three members of a criminal group behind the March 29 terrorist attacks on the Moscow metro have been killed, Federal Security Service chief Alexander Bortnikov said on Thursday.

The three killed were members of a "criminal group, which included people who had accompanied the suicide bombers on their way to Moscow and one of them to the site of the terrorist attack," he said at a meeting with President Dmitry Medvedev.

"Unfortunately, we did not manage to detain them alive, they offered fierce armed resistance and were eliminated," he added.

Twin powerful blasts hit the Moscow metro during the morning rush hour on March 29, killing 40 people and injuring over 100. Medvedev urged a prompt investigation into the tragedy and said those involved in the bombings should be severely punished.

Bortnikov said the investigation had now established the identities of everyone involved in organizing and committing the attacks.

The Russian president said the investigation should be continued in line with Russian criminal law.

"Those [terrorists] who offer resistance should be eliminated, have no pity," he said.

Russia has been fighting militants in the volatile North Caucasus region for almost two decades and has carried out two brutal separatist wars in Chechnya. Analysts have suggested that the terrorist attacks were revenge for a recent operation in Chechnya that led to the deaths of over 20 radical Islamic militants.

The Russian government has recently made moves to improve social and economic conditions in the North Caucasus to prevent locals from joining terrorist groups.

GORKI, May 13 (RIA Novosti)