MOSCOW -- Three rights groups have filed a legal case in Moscow against the Wagner Group -- mercenaries who work at the behest of Russian President Vladimir Putin -- over the torture of a detainee in Syria.

The legal push against the private military contractor follows a wave of torture-related cases in Europe against Syrian regime officials a decade into a punishing war whose tide was turned by Moscow's military intervention in 2015.

The Paris-based International Federation for Human Rights, Russian advocacy group Memorial and the Syrian Centre for Media and Freedom of Expression on Monday (March 15) filed the criminal complaint against alleged members of the Wagner Group over the 2017 beheading of a man believed to have deserted from the Syrian army.

The rights groups said in a statement they had filed evidence that clearly links at least one defendant to Wagner.

In video clips of the gruesome killing, four Russian mercenaries operating in Syria as part of the Wagner Group are seen murdering a young Syrian man.

The men break the victim's legs with a sledgehammer, crush his chest, behead him, chop off his arms and hang his corpse by the legs, before setting his body on fire.

The victim hailed from the town of al-Kharita in Deir Ezzor, according to Syrian journalist Mohammed al-Abdullah.

He had completed his compulsory service in the Syrian army and then travelled to Lebanon to work as a daily wage worker. Upon his return to Syria, he was led away to serve reserve duty at Tiyas air base (T-4) in Homs province.

He escaped, but was captured by the Russian mercenaries in the area of al-Shaer oil field, and met his fate at their hands, al-Abdullah said.

'Wave of impunity'

Yevgeny Prigozhin, a Saint Petersburg businessman and Putin ally who made a fortune in the catering business by signing lucrative contracts with Russia's military and government, is widely recognised as the bankroll behind the Wagner Group.

This private funding allows the Kremlin to advance its interests and carry out its hybrid warfare under a cover of plausible deniability.

Prigozhin, who has been hit with US sanctions for meddling in the US presidential election in 2016, denies any association with Wagner, and the case probing potential "war crimes" is unlikely to lead to any convictions.

But the proceedings represent a rare attempt to bring the Wagner Group out into the open, several years after reports of its deployment in conflicts across the Middle East and Africa first emerged.

"This complaint is important because we aren't just dealing with a single crime. This is a whole wave of impunity," said Alexander Cherkasov, a senior member of Memorial.

"People who escape punishment after carrying out crimes like this are given the opportunity to repeat them in places like Chechnya, eastern Ukraine and Syria," he said. "In the end they come back to Russia and walk on the streets among us."

'Legal and moral responsibility'

Although private military companies are illegal in Russia, the Wagner Group has in recent years played an increasingly important role in buttressing and realising the Kremlin's ambitions abroad, observers say.

Members of the group were dispatched alongside Russian warplanes and ground troops following Moscow's intervention in September 2015 on the side of Syrian President Bashar al-Assad's army.

Moscow has never confirmed reports of Wagner mercenaries but on Monday said that since its operation launched in Syria, 112 Russian troops had died in combat operations.

Wagner's presence there was forced into the spotlight in 2018 when independent newspaper Novaya Gazeta reported on the 2017 torture and slaying that are the object of the NGOs' criminal complaint.

The complaint brought Monday on behalf of the victim's family aims to force Moscow to bring criminal proceedings against the alleged members of the private contractor group, in what NGOs say is the first case of its kind.

Later on Monday, Novaya Gazeta said its offices in Moscow had been targeted that morning in a "chemical attack" that the paper linked to its reporting on the detainee.

The complainants are seeking an admission of legal and moral responsibility.

"The Russian government must assume its legal and moral responsibilities for the violations committed by its army, including the private entities involved in external military operations under its command, such as the Wagner Group," said Mazen Darwish, director general and founder of the Syrian Centre for Media and Freedom of Expression.

Ilya Novikov, one of the Syrian family's lawyers, said Russia is obliged under its constitution to investigate crimes carried out by Russian nationals abroad.

But "the Investigative Committee has not, to date, initiated any investigation of the killing," said Novikov.

The Kremlin Monday said it was aware of the complaint but that it was "an issue for the Investigative Committee, not the presidential administration".

The lawsuit follows dozens of other cases brought in Germany, Austria, Sweden and Norway against Syrian regime officials by about 100 refugees, backed by the European Centre for Constitutional and Human Rights, a Berlin-based group.

Across Europe, activists are joining forces with police and United Nations (UN) investigators in collecting testimonies, sifting through tens of thousands of photos, videos and files of one of the best documented conflicts in history.

European officials have taken note of Wagner's role in conflicts beyond Syria, slapping sanctions on Prigozhin last year for the group's role in destabilising Libya.

Separately, Britain on Monday announced targeted sanctions against six members of the Syrian government, including Foreign Minister Faisal Mekdad, for "repressing the Syrian people or benefitting from their misery".

The measures also target an al-Assad adviser, two senior military commanders and two businessmen.

London said it was also working through the UN Security Council to push Syria "to engage meaningfully" in a UN-led peace process and constitutional committee talks in Geneva.

It urged Damascus to release those held in arbitrary detention, and allow unhindered aid access throughout the country, the British Foreign Office said.