Scores of Russian mercenaries were reportedly killed when US forces carried out air strikes against a pro-regime force in eastern Syria last week, according to US and Russian reports.

If the high estimates of Russian casualties are confirmed, it would be the most lethal clash between US and Russian citizens since the end of the cold war, and it comes at a time when proxy forces in Syria are increasingly coming into contact, as they compete for territory vacated by retreating Islamic State militants.

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Bloomberg News quoted Russian sources as saying as many as 200 Russian nationals could have been killed in a clash last Wednesday and Thursday around the oil fields of Deir Ezzor region between forces loyal to Bashar al-Assad and a Kurdish-led opposition force supported by US advisers.

The New York Times said that Russian and Syrian officials had estimated that dozens of Russians had been killed.

US Central Command said on Tuesday that US forces came under fire from a tank, and that they responded with three hours of intense strikes from drones and B-52 bombers against the attackers, which it described as a unit of 300 to 500 fighters.

Air force Lt Gen Jeffrey Harrigan said US advisers had been in an operating base alongside units of the mostly-Kurdish Syrian Democratic Forces (SDF) in the course of anti-Isis operations, when they came under tank fire. He added that the senior officer at the base called in airstrikes in response.

Harrigan said the attacking forces were fighting on the side of Bashar al-Assad’s regime, but said he would “not speculate on the composition of this force, or whose control they were under”.

He said: “This is a hostile force that was coming after our partners and coalition advisers so we are going to defend ourselves.”



The Conflict Intelligence Team, a group of well-respected Russian online researchers, said it had found three Twitter posts announcing the deaths of Russian mercenaries in Syria on 7 February.

“It is beyond doubt that this incident indeed took place. It is notable that the posts about their deaths emerged rather quickly – relatives and colleagues often learn of Russian mercenaries’ deaths weeks or even months after the fact,” the group said in a post.

Grigory Yavlinsky, an opposition politician, said that if Russian citizens had been killed in large numbers, Vladimir Putin and other officials owed the country an explanation.

“I demand an explanation as to why Russian nationals take part in ground military operations in Syria, despite the statements by the president and defense minister that Russian military formations will be withdrawn from this country,” Yavlinsky said.

Timeline Syria's road to destruction January 2011 The toppling of Tunisia’s president, Zine al-Abidine Ben Ali, after mass protests marks the beginning of the Arab spring. Revolutions in Yemen, Egypt, Libya and elsewhere ensue.

February 2011 Children spray slogans attacking President Bashar al-Assad on a wall in the Syrian town of Deraa . A brutal police response prompts peaceful street protests, which spread.

July 2011 The opposition to the regime takes up arms after Assad refuses concessions. The Free Syrian Army is formed, backed by armed forces defectors.

2012 - 2013 More armed groups, including the al-Qaida-linked al-Nusra Front, join the fight. As the opposition fragments, neighbouring states intervene, backing rival rebel forces.

- August 2013 Civilians in east Ghouta are attacked with sarin gas. Th e UK parliament votes against military action. US leader Barack Obama ignores his own red line banning chemical weapons use and declines to intervene.

September 2013 The US accepts a Russian proposal to remove Assad’s chemical weapons stockpile, ostensibly to stop future attacks with the munitions, which are internationally banned.

2014 - 2016 Europe faces an unprecedented influx of millions of Syrian refugees. Half the country’s 23 million population is displaced and in need. The influx causes an anti-immigrant backlash, notably in Germany, which controversially opens its borders.

- 2014 Islamic State (Isis) achieves dramatic territorial gains in Syria and Iraq, seizing the cities of Raqqa and Mosul, killing thousands and laying waste to cultural heritage. It declares a caliphate and jihad against the western powers and their allies.

September 2015 Having provided diplomatic cover since the war began, Russia intervenes militarily at Assad’s request . The intervention, supported by ground troops supplied by Iran, turns the tide in the regime’s favour.

December 2015 MPs in the UK vote to join a US-led European and Arab military coalition to fight an air war against Isis in Syria and Iraq. The coalition forms an alliance with the Kurdish peshmerga. There is however little appetite for deploying western ground forces.

2016 - 2017 Backed by Russian fighter jets, the Assad regime increasingly targets civilian areas where it claims that rebel forces are based. Hospitals, schools and clinics are routinely hit. As a result, civilian casualties grow steadily across the country. There are also reports of more chemical weapons attacks despite the 2013 agreement to end their use in the country.

- December 2016 The five-year siege of Aleppo ends with victory for the regime.

October 2017 Isis is defeated and expelled from Raqqa. But as the year ends, there are reports of the Islamists regrouping in Anbar, Iraq, and of a growing al-Qaida presence in north-west Syria.

January 2018 The collapse of Russian-sponsored peace talks in Sochi prompts an all-out regime offensive against rebel strongholds, with mass casualties and further displacement. Turkish troops cross the border into northern Syria and enter the Kurdish held enclave of Afrin. Show Hide

Without mentioning the incident directly, Russia’s foreign minister, Sergey Lavrov, warned on Tuesday that “Americans have taken dangerous unilateral steps” in Syria in siding with SDF forces, who control an extensive region in northern Syria.

“Those steps look increasingly as part of efforts to create a quasi-state on a large part of Syrian territory from the eastern bank of the Euphrates all the way to the border with Iraq,” Lavrov said, in remarks that highlighted the geopolitical jostling in Syria in the wake of the fight against Isis.

Igor Strelkov, a nationalist army veteran who was instrumental in Russia’s military operations in eastern Ukraine but now opposes the Kremlin, said in an online post that the deaths should be a warning for any Russians considering “going to Syria to earn [money]”.