Six people have been arrested in St Petersburg on suspicion of recruiting for Isis and involvement in terrorist activity, investigators have said.

The suspects are from Central Asian countries and the group of six men has been active since 2015, said the Investigative Committee of the Russian Federation in a statement. It also said the group had been involved in recruiting for former al-Qaeda affiliate in Syria, Jabhat al-Nusra Front.

Investigators said it is not clear yet if they are connected to Akbarzhon Jalilov, a Kyrgyzstan man suspected of carrying out a bomb attack on a St Petersburg Metro train on Monday, which killed 14. Jalilov had lived in Russia for several years and was a Russian citizen.

Russia's anti-corruption agency said that those detained had been recruiting other Central Asian immigrants in St Petersburg "to carry out terrorist crimes" and join "illegal armed groups" such as Isis.

Investigators said that extremist literature had been discovered in a search of the living quarters of the men.

Investigations are continuing into the Metro attack, with police probing Jalilov's background and whether he had links to extremist groups. No group has claimed responsibility for the attack.

The head of the anti-corruption agency, Alexander Bastrykin, has ordered officials to investigate potential links between Jalilov and Isis.

Isis has reportedly recruited thousands of militants from Central Asian former Soviet Republics.