Security has been tightened across Russia after two powerful blasts tore through the Moscow metro system on Monday morning, killing at least 37 people.

"It is difficult to prevent such terrorist attacks and to provide security on transport," President Dmitry Medvedev said. "It is necessary to tighten up what we do and to look at the problem on a national scale.

"Obviously, what we have done before is not enough," he said.

Interior Ministry head Rashid Nurgaliyev told Medvedev that security was being boosted across the country, "in particular in those cities with metro systems."

The first attack took place at 7:52 a.m. (03:52 GMT) at the Lubyanka station, located a short distance from Red Square and the headquarters of the Federal Security Service (FSB), and killed at least 24 people

The second blast detonated some 40 minutes later at the nearby Park Kultury station, also within walking distance of the Kremlin. At least 12 people lost their lives.

One more person is reported as having died in the bombings. The total number of injured in both blasts is over 60.

The head of the Federal Security Services (FSB) said terrorists from Russia's North Caucasus may have been involved in the attacks.

Russia's top investigator Vladimir Markin said police may be able to identify the two female suicide bombers who carried out the attacks. He added that the women's faces had not suffered in the blasts.

Markin earlier told journalists that the second attack had been carried out by a "dark-haired woman" and that "fragments of her body" found at the scene suggested she had had the equivalent of 1.5 kg of TNT strapped to her waist.

A police source earlier told RIA Novosti that "An inspection of the scene indicates that the bomb was detonated at a height of 100-200 cm and was apparently attached to the waist of a female suicide bomber."

Russia has been fighting militants in the North Caucasus for over a decade, including two brutal separatist wars in Chechnya. Analysts suggest Monday's attacks are revenge for a recent operation in Chechnya that saw the deaths of over 20 radical Islamic fighters.

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)