Several suspects in the murder of Russian opposition politician Boris Nemtsov have been identified, according to the head of the Russian security service.

FSB boss Alexander Bortnikov admitted that an unspecified number of suspects are being considered by authorities, according to Russia's Tass news agency. But he remarked that the suspects are part of only one version of events that they are considering.

Nemtsov was shot dead in Moscow on a bridge overlooking the Kremlin on 27 February. A fierce Vladimir Putin critic, the politician was working on a report on Russia's alleged direct involvement in the eastern Ukraine conflict - something the Kremlin has long denied.

The development came as a car sought in connection with the Nemtsov murder has reportedly been found to belong to a federal state enterprise called FSUE, which provides services to the Finance Ministry, Goznak (the Russian state body printing banknotes) and other agencies.

The Finance Ministry spokesperson told Tass that the Ford car "is a vehicle of an in-house security service".

Putin's opponents say Nemtsov's murder was political and the president is directly responsible for it. Authorities and state-controlled media have instead argued the killing was a provocation aimed at destabilising Russia by tainting its leader's reputation.