Five men accused of murdering Boris Nemtsov, a Russian opposition leader, are standing trial in Moscow.

Nemtsov, a 55-year-old former deputy prime minister, was killed in February last year after being shot at least four times in the back while crossing a bridge near the Kremlin.

At the time, he was working on a report that he said proved Russia's direct involvement in a separatist rebellion that has raged in eastern Ukraine since April 2014. Moscow has denied the accusations.

Details in the case have so far remained vague despite a pledge from Russian President Vladimir Putin to pursue the killers vigorously.

The five men, who have been identified as ethnic Chechens, are accused of murder and arms trafficking and could face jail terms of up to eight years.

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Media reports previously identified them as Anzor Gubashev, Shagid Gubashev, Zaur Dadayev, Ramzan Bakhayev and Tamerlan Eskerkhanov. Some of them served in the military.

Al Jazeera's Rory Challands, reporting from outside the court in Moscow, said the location and set-up of the trial was unusual.

"The arrests came fairly swiftly after the murder and confessions also followed. However, those confessions have since been retracted and those men are now pleading their innocence," Challands said.

"This is an unusual trial in many ways. First, it is being held in a military court. Second, this is also a jury trial and that is unusual in Russia. Most people stand trial before a judge and that has also provided a problem and is why this trial has been delayed."

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Nemtsov's killing shocked Russia's already beleaguered and marginalised opposition. Suspicion in the opposition is rampant that the killing was ordered by the Kremlin in retaliation for his criticism of Putin.

Russian officials have denied involvement in Nemtsov's death. Putin has called  the incident a "provocation", suggesting Nemtsov was killed by one of Putin's enemies to stoke discontent in Russia.

Another theory presented by Russian police is that Nemtsov may have been targeted for his reported support of the French satirical magazine Charlie Hebdo, which was attacked one month before his death for its controversial depictions of the Prophet Muhammad.

Source: Al Jazeera News And Agencies