Russian mercenaries are reported to be in the thick of the fighting in Syria, helping President Bashar al-Assad's forces.
One shadowy group in particular, called Wagner PMC (Private Military Company), hit the headlines because of a clash on 7 February that resulted in dozens of Russian casualties.
The Wagner Group attracted worldwide attention after some of its personnel in Syria were killed and wounded in a devastating 7 February air and artillery strike by the US military targeting pro-Syrian government militia forces.
The toll of dead and wounded is disputed, but it is still not clear why Russian irregulars attacked a base held by Kurdish anti-Assad forces and where US advisers were present. US forces retaliated with air strikes.
Mercenary forces are illegal in Russia, yet there is evidence the government established and supported Wagner's early growth, operations in eastern Ukraine's Donbas region, and their operations in Syria, says a report by Polygraph.info.
Estimates of the number of Russian citizens killed in the 7 February incident range from five to more than 200, according to Wagner personnel.
The Conflict Intelligence Team, an NGO that investigates reports of military activity in Ukraine and Syria, has confirmed the deaths of ten ''contractors'' - nine Russians and one Ukrainian.
Russian officials from the Kremlin, foreign ministry, and defence ministry initially denied or downplayed the number of Russian victims. Statements have been contradictory, at times accusing the press of spreading ''disinformation'' and other times admitting higher casualty figures while still denying any knowledge of the Russian citizens' activities in Syria.
This week, the Russian foreign ministry admitted there were ''dead citizens of Russia and the CIS (Commonwealth of Independent States) countries'' and "several dozen" wounded in the clash.
''Our guys were going to commandeer an oil refinery and the Yankees were holding it,'' said a paramilitary fighter in an audio recording obtained by Polyraph.info from a source close to the Kremlin and published 16 February.
''So they tore us to pieces for sure, put us through hell, and the Yankees knew for sure that the Russians were coming,'' the unidentified soldier said.
In June 2017 the US Treasury added Wagner PMC to a long list of Russian individuals and entities subject to sanctions because of their involvement in the Ukraine conflict.
The PMC "has recruited and sent soldiers to fight alongside separatists in eastern Ukraine", the US Treasury said.
The US also identified Dmitry Utkin as Wagner's "founder and leader" and placed him on the list.
Russian media reports, quoting anonymous military sources, reveal that Utkin earlier served in a special forces brigade of Russian military intelligence, the GRU. Then in 2013 he went to Syria with a group of fighters recruited by a company called "Slav Corps", the BBC reports.
The GRU secretly oversees Wagner, according to security sources quoted by Russian RBC news. Russia's official military deployment in Syria began in September 2015; it has mostly taken the form of air strikes, sometimes hitting civilian areas hard.
Wagner is estimated to have as many as 2,500 men in Syria. Its officers serving in Syria are reported to earn up to 300,000 roubles (£3,800; $5,300) a month.
According to media analysis by BBC Monitoring, Wagner uses a training base at Molkino, in the Krasnodar region of southern Russia, not far from eastern Ukraine.
The reports say Wagner was involved in Russia's military takeover of Ukraine's Crimea peninsula in March 2014 and in the pro-Russian insurgency which erupted in Ukraine's Donetsk and Luhansk regions the following month.
The mercenaries can be deployed in especially tough ground fighting alongside Syrian government troops. And as the Russian government does not officially recognise the mercenaries' existence it can deny or play down any Russian casualties.
Their role has been compared to that of US military contractors in Iraq, who were deployed on a large scale.
The US suffered heavy casualties in Vietnam, and later the Afghanistan war was very costly for Russia. Those wounds - and the associated public anger - encouraged both countries to privatise war in recent years.
Military sources quoted by RBC said that when Russia helped Assad forces to recapture Palmyra it was the Wagner men who went in first. "First Wagner's guys go to work, then the Russian ground units come in, and then the Arabs and the cameras," an unnamed ex-Wagner officer said.