Tens of thousands of mercenaries are believed to be fighting for Russia in Ukraine.
They belong to an organisation called the Wagner Group, which has been used in Russian military operations in many countries.
"Wagner almost certainly now commands 50,000 fighters in Ukraine and has become a key component of the Ukraine campaign," according to the UK Ministry of Defence.
It says the organisation started recruiting in large numbers last year because Russia had trouble finding people for the regular army.
About 80% of its troops in Ukraine have been drawn from prisons, according to the US National Security Council.
Before Russia's invasion of Ukraine, it is thought the Wagner Group had only about 5,000 fighters.
They were mostly experienced former soldiers – many of them from Russia's elite regiments and special forces.
Although mercenary forces are illegal in Russia, the Wagner Group registered as a company in 2022 and opened a new headquarters in St Petersburg.
"It is openly recruiting in Russian cities, on billboards, and is being named in Russian media as a patriotic organisation," says Dr Samuel Ramani, of the Royal United Services Institute think tank.
The Wagner Group has been heavily involved in Russian efforts to capture the city of Bakhmut, in eastern Ukraine.
Ukrainian troops say Wagner fighters have been sent into attacks in large numbers over open ground, with many killed as a result.
After Russia claimed to have captured the town of Soledar, near Bakhmut, a row broke out between its defence ministry and the Wagner Group over who should get the credit.
At first, the defence ministry did not mention that the Wagner Group was involved in the fighting. However, it then conceded that its mercenaries had played a "courageous and selfless" role.
A BBC investigation into the Wagner Group highlighted the believed involvement of a former Russian army officer, Dmitri Utkin.
A veteran of Russia's wars in Chechnya, he is thought to have been Wagner's first field commander and to have named it after his former radio call sign.
The head is Yevgeny Prigozhin, a rich businessman nicknamed "Putin's chef" because he provided catering for the Kremlin.
The Wagner Group's first operation was helping Russia annex Crimea in 2014, says Tracey German, professor of conflict and security at King's College London.
"Its mercenaries are thought to have been some of the 'little green men' who occupied the region," says Prof German, referring to the troops who appeared on the streets of Crimea in uniforms without any identifying markings.
In the weeks before Russia's invasion of Ukraine, it is thought Wagner carried out "false flag" attacks to give the Kremlin a pretext for attacking.
Since 2015, Wagner Group mercenaries have been in Syria, fighting alongside pro-government forces and guarding oilfields.
There are also Wagner Group mercenaries in Libya, supporting the forces loyal to General Khalifa Haftar.
The Central African Republic (CAR) has invited the Wagner Group to guard diamond mines, and it is thought to be guarding gold mines in Sudan.
The government of Mali, in West Africa, is using the Wagner Group against Islamic militant groups.
Yevgeny Prigozhin is thought to make money from Wagner Group operations abroad.
The US Treasury says he uses its presence to enrich mining companies which he owns and has placed them under sanctions.
An unnamed White House spokesman told Reuters that Mr Prigozhin may want the Wagner Group to capture Bakhmut so he can control salt and gypsum mines in the area.
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Russian Mercenaries: Inside the Wagner Group
In January, a former commander claimed asylum in Norway after deserting from the mercenary outfit. He claims to have witnessed war crimes in Ukraine.
Three Wagner Group mercenaries are alleged by Ukrainian prosecutors to have killed and tortured civilians near Kyiv in April 2022, alongside regular Russian troops.
German intelligence says Wagner mercenaries may also have massacred civilians in Bucha in March 2022.
The United Nations and the French government have accused Wagner mercenaries of committing rapes and robberies against civilians in the Central African Republic.
In 2020, the United States military accused Wagner mercenaries of having planted landmines and other improvised explosive devices in and around the Libyan capital, Tripoli.
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