At least six Russian nationals, and potentially dozens more, are believed to have been killed in fighting between pro-government forces and the US-led coalition in Syria - in one of the most significant clashes between the two nations since the Cold War.

The coalition had said that their forces had come under an “unprovoked attack” in territory held by the US-backed Syrian Democratic Forces (SDF) in eastern Deir Ezzor province, from what it said were “pro regime” forces loyal to Syrian President Bashar al-Assad. More than 100 fighters said to have been killed out of an attacking force of around 500 during the clashes on 7/8 February. Colonel Thomas F Veale, a US military spokesman said that the coalition forces had acted in “self defence” and “targeted the aggressors with a combination of air and artillery strikes”.

Then, on Saturday, a US military drone aircraft destroyed a Russian-made T-72 tank near the area around Al Tabiyeh. The US military said it destroyed the tank after it moved within firing range of US-backed forces.

The coalition would not confirm the death toll or a Russian presence when asked about the 7/8 February attack by The Independent, but reports have surfaced in Russia that soldiers from the country had come under fire.

Most, if not all of the soldiers were affiliated to the pseudo-mercenary Wagner military group. This group, named after the retired Russian officer that leads it, is linked to Russian military and security forces and has operated as an arms-length formation in both eastern Ukraine - although Russia has denied direct military involvement in the conflict there - and Syria.

Data on the numbers of dead Russian soldiers varies. Some unverified accounts suggest hundreds, but this is likely to be an exaggeration. Ruslan Leviev, lead researcher at Conflict Intelligence Team (CIT), has investigated the battle in forensic detail. He says his group has managed to verify six Russian deaths.

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“The number will probably grow to two or three dozen, but we don’t believe numbers of 200 dead are credible,” he told The Independent.

Russian media have already reached the widow of one of the soldiers identified in CIT’s research. In an interview published in Znak.ru, a news portal based in Russia’s third city of Yekaterinburg, Yelena Matveyeva confirmed the death of her husband Stanislav Matveyev, 38. She told journalists said his body was due to arrive in the southern town of Rostov-on-Don on Tuesday.

The death of Russians under direct American, or at least US-led fire would seem a very serious matter. But the response in Moscow has been muted, a reflection of the precarious situation both nations now found themselves in. With the territory of Isis shrinking as they are pushed out, it leaves all of the various nations now acting in Syria without a common enemy and increases the possibility of clashes.

Syria, entering its seventh year of civil war, is now a patchwork of competing claims and forces from around the globe. Mr Assad is backed by Russia, as well as Shia militia groups themselves backed by Iran, while the US is supporting the Kurdish SDF forces who have been a crucial ally in battling Isis – the major aim in Syria for the administration of President Donald Trump. Turkey has also started an offensive to push the US-backed Kurdish forces out of the northern town of Afrin, near the Turkish border, which has also increased tensions between Washington and Ankara. Israel has also warned the Syrian government over the presence of Iran after one of its jets was recently shot down over Syrian territory.

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The Pentagon says it is in constant contact with Russian forces on the ground as part of an existing de-confliction agreement that had been agreed between Washington and Moscow. The attack last week, and the incursion on Saturday were said to be in violation of a de-confliction area around the Euphrates, with the coalition saying it had “been observing a slow build-up of personnel and equipment” over “a week”. US Defence Secretary James Mattis, said that he had read the reports about the Russian deaths, but that "we have not received word" confirming anything. Talking about the striking of the tank on Saturday, he added: “This could just be a local couple of guys doing something. I don't want to dignify it as a big attack.”

Russia’s Special Representative to the UN Vasily Nebenzya condemned the attack but made no mention of the Russian deaths. The Ministry of Defence meanwhile denied that the Syrian operation had been coordinated with Russian military command. It also stated that no Russian soldiers were present in the area. Wagner’s unofficial status made such a legalistic statement possible. It also made Russian protest difficult.

"We don't have information about other Russians who might be in Syria," Kremlin spokesman Dmitri Peskov said on Tuesday, saying that the only records Russia had were of members of the country's armed forces.

However it is has not stopped the two nations hitting out at the motives behind the attacks. The coalition said that the pro-regime forces were “likely seeking to seize oilfields” in the resource-rich Deir Ezzor, while Russian Foreign Minister Sergei Lavrov said on Tuesday the “Americans have taken dangerous unilateral steps.”

Vladimir Frolov, a security expert and former Kremlin adviser, told The Independent Russia almost certainly knew about Wagner’s participation in the Syrian-led attack. “My hunch is both the Syrians and the Russians thought they could get away with it and grab some oil assets with the SDF distracted by the Turkish offensive, but miscalculated,” he said.

Presidents Putin and Trump and Putin spoke by phone in an unscheduled call on Monday, and it is likely that the issue was discussed. “I strongly believe they decided to... not blow it up into a bigger scandal,” Mr Frolov told the Independent.