A US air and artillery strike has killed Russian combatants in the first lethal violence in Syria between the two nuclear powers, according to sources on both sides. The battle, which was briefly alluded to in a US-led coalition statement last week, took place in the oil-rich Deir Ezzor province in eastern Syria. On 7 February, a large force loyal to Bashar al-Assad and supported by tanks and artillery advanced and fired at a Syrian Democratic Forces base manned by Kurdish troops and American military advisors, a US military spokesman said in a statement to Bloomberg on Tuesday. The United States, which was communicating with the Russian side during the clash, drove the attackers back with aircraft and artillery fire, suffering no fatalities, the spokesman said. On 10 February, a US drone destroyed an advancing Russian-made T-72 tank from the “same hostile force,” the US military said on Tuesday. While reports have varied widely, claiming anywhere from a handful to more than a hundred Russians were killed and describing them alternately as military troops or private contractors, the 7 February clash nonetheless appears to have been the deadliest between US and Russian citizens since the Cold War. A US military adviser shakes hands with a Kurdish commander in Manbij in northern Syria last week Credit: Susannah George/AP Photo On Monday, the Russia-based independent research group Conflict Intelligence Team published the names of four Russians who had been killed by the US strike. It said the men were mercenaries from the Wagner group, a highly secretive private military company whose alleged commander was photographed with Vladimir Putin in 2016. Friends and relatives confirmed to RBC newspaper that the men had been killed in Syria on 7 February. Conflict Intelligence Team told The Telegraph on Tuesday that three other Russians were also killed in the attack, which it said was the only time Russians had been killed by the Western coalition. The independent newspaper Novaya Gazeta reported that 13 Russians had been killed and 15 wounded in the strike. It said Wagner troops had been operating with a special forces unit known as the “ISIS Hunters”. Bashad regime soldiers wounded in the US strikes near Deir Ezzor are seen in a hospital last week Credit: AFP/Getty Images Igor Strelkov, a nationalist with links to Russian intelligence who commanded Russia-backed separatists in eastern Ukraine, said 100 Wagner employees had died in the strike. Bloomberg quoted Russian sources as saying that 200 professional soldiers, most of them Russian, were killed, while an American official told the publication about 100 had been killed. If true, those numbers would easily eclipse previous Russian losses in Syria, which has been presented by Mr Putin as a largely bloodless conflict. Russia has insisted it does not have troops on the ground even as reports have mounted of small numbers of soldiers and mercenaries killed. Surveys have shown Russians are largely lukewarm toward the Syrian conflict. Vladimir Putin speaks to servicemen at Russia's airbase in Syria in December Credit: Mikhail Klimentyev/Pool Photo via AP In other circumstances, such a clash would have likely sparked a diplomatic crisis, but the Kremlin did not appear to want to discuss possible casualties before Mr Putin stands for re-election next month. After liberal presidential candidate Grigory Yavlinsky called on the president to comment on the Russian deaths, Mr Putin's spokesman said these reports “need verification” and argued that so many Russians were located in so many countries it was “difficult to have any detailed information”. In a statement last week, the defence ministry said no Russian soldiers were in that area of Deir Ezzor and claimed that the US strike had hit Syrian rebels by mistake, injuring 25 of them. A memorial ceremony for Russian air force pilot Roman Filipov, who killed himself with a grenade after his aircraft was shot down over Syria this month Credit: Vadim Savitsky/Handout via Reuters “If public opinion paints a picture for itself that the Syrian war will require losses and those to blame are not terrorists but American soldiers, then they will have to react, and no one wants to react to this right now,” said Carnegie Centre Moscow analyst Alexander Baunov. The clashes in Deir Ezzor bode ill for the future of the war, suggesting that conflicts between the many regional powers present in Syria could grow more frequent even as the terrorists are defeated. “The fight with the Islamic State is being replaced by old and new conflicts amid the intersection of internal and external players' interests,” said Conflict Intelligence Team researcher Ruslan Leviev.