Russia has identified two suicide bombers responsible for attacks that killed 34 people in the city of Volgograd last month and arrested two suspected accomplices in violence-torn Dagestan province, officials said on Thursday.

The National Anti-Terrorism Committee said the bombers, whose attack raised fears of further violence before the Sochi Winter Olympics next week, were members of a militant group in Dagestan in the restive North Caucasus.

A bombing at the railway station in Volgograd on Dec. 29 was followed a day later by a blast that ripped apart a trolleybus in the city 700 km northeast of Sochi, where the Olympics start on Feb. 7.

The blasts were the deadliest attacks in Russia outside the North Caucasus, the cradle of an Islamist insurgency whose leader has urged fighters to prevent the Olympics going ahead, since a bomber killed 37 people at a Moscow airport in 2011.

The National Anti-Terrorism Committee identified the bombers as Asker Samedov and Suleiman Magomedov, called them members of the "Buinaksk Terrorist Group", and said it had known their names for some time.

Buinaksk is a city in Dagestan.

Two brothers suspected of helping send the bombers to Volgograd were detained in Dagestan on Wednesday, the committee also said. It identified them as Magomednabi and Tagir Batirov and said the investigation was continuing.

A video posted on the Internet last week by a group identifying itself as Vilayat Dagestan featured what it said were the Volgograd bombers donning explosive belts and warning President Vladimir Putin to expect a "present" at the Olympics. The video named the men only as Suleiman and Abdulrakhman.