Boris Nemtsov murder rally
Supporters at a rally held in memory of Russian opposition politician Boris Nemtsov. (Getty)

Two more men have been arrested over the murder of Russian opposition leader Boris Nemtsov, who was gunned down in Moscow on 27 February.

The former deputy prime minister and fierce critic of Russian President Vladimir Putin was shot four times in the back as he walked with his girlfriend along a bridge in the heart of the Russian capital.

Initial reports said two suspects had been arrested on Saturday (7 March), with state news agencies Tass and RIA Novosti reporting that two more men had been arrested later the same day. The latest two suspects are from Chechnya and have been held in the neighbouring republic of Ingushetia.

Their names have not been released, but the agencies reported that one is the younger brother of Anzor Gabashev, who was arrested alongside Zaur Dadayev, also from the North Caucasus region, on Saturday.

The same day, Ingush Security Council chief Albert Barakhoev announced that Anzor Gubashev worked for a private security company in Moscow, and Dadayev served in a battalion attached to the Chechen interior ministry.

According to the AP, Barakhoev said that two other men had been seized at the same time, and Dadayev's mother was quoted by the Interfax news agency as saying the other two men were her nephews.

The head of Russia's Federal Security Service (FSB), Alexander Bortikov, announced the detention of two men on Saturday. He did not provide any details of how and where they were being held, but said the investigation was ongoing. The men have not been formally charged, and no announcement has been made concerning their possible motive.

Nemtsov's assassination shocked Russia and has sparked international condemnation.

His supporters believe his killing may have been ordered by the Kremlin in order to silence dissent. Nemtsov, 55, was an outspoken critic of government policies and of Russia's involvement in the Ukraine.

These allegations have been strenuously denied by the government. President Putin has called the killing a "provocation", and vowed that those who committed the "shameful and tragic" murder would be brought to justice.

On Sunday (8 March) AP reported that a spokesperson for a court in Moscow said that five suspects involved in Nemtsov's murder are to be arraigned for trial.

The fifth suspect has not been identified.