Six female suicide bombers preparing attacks in central Russia have been arrested in Dagestan, the National Anti-Terror Committee said.

The women, who were aged 15 to 29, were the widows of Islamic militants killed by the authorities, the committee said in a statement late Monday.

It was unclear how close the women were to carrying out attacks, but the statement said they had written good-bye letters to their families.

"Officers seized the detained suspects' farewell letters to their families, expressing their wish to end their lives and advising their 'sisters' to follow suit," it said.

A video played on state television showed two handguns, a silencer, a belt for explosives and three grenades. A young girl in a gray headscarf was shown describing how she was trained to use grenades.

The six women were among eight people detained in a house in Makhachkala, the capital of Dagestan, the committee said.

One of the other detainees arranged for the transportation and lodging of the two female suicide bombers who attacked the Moscow metro on March 29, killing 40 people, it said.

"One of the detained men delivered the suicide bombers who committed terrorist acts at the Lubyanka and Park Kultury metro stations in March of this year," the statement said.

Authorities have said both women who blew themselves up in the Moscow attack were natives of Dagestan, which has eclipsed nearby Chechnya as a center of rebel activity.

Meanwhile, gunmen have killed a policeman who earlier had lost his father, mother and sister to attackers in Dagestan, local Interior Ministry spokesman Vyacheslav Gasanov said Tuesday.

Police Lieutenant Rasul Magomedov was gunned down Monday afternoon outside a village police station, he said.

Magomedov's father, a district police commander, was killed by insurgents in 2008. The next year, his mother and sister were killed in a bomb blast at the cemetery where they were visiting the father's grave.

Also Monday, police investigator Akhmad Gadzhiev was shot and killed while driving in the Khasavyurt area near the border with Chechnya, according to another ministry spokesman, Magamet Tagirov.

(Reuters, MT, AP)