A team of about 30 Delta Force soldiers and Rangers from the Joint Special Operations Command were working alongside Kurdish and Arab forces at a small dusty outpost next to a Conoco gas plant, near the city of Deir el-Zour

The artillery barrage was so intense that the US commandos dived into foxholes for protection, emerging covered in flying dirt and debris to fire back at a column of tanks advancing under the heavy shelling. It was the opening salvo in a nearly four-hour assault in February by around 500 pro-Syrian government forces – including Russian mercenaries – that threatened to inflame already simmering tensions between Washington and Moscow.

In the end, 200 to 300 of the attacking fighters were killed. The others retreated under merciless air strikes from the US, returning later to retrieve their battlefield dead. None of the Americans at the small outpost in eastern Syria – about 40 by the end of the firefight – were harmed.

The details of the 7 February firefight were gleaned from interviews and documents newly obtained by The New York Times. They provide the Pentagon’s first public on-the-ground accounting of one of the single bloodiest battles the US military has faced in Syria since deploying to fight the Isis.

The firefight was described by the Pentagon as an act of self-defence against a unit of pro-Syrian government forces. In interviews, US military officials said they had watched – with dread – hundreds of approaching rival troops, vehicles and artillery pieces in the week leading up to the attack.

The prospect of Russian military forces and US troops colliding has long been feared as the Cold War adversaries take opposing sides in Syria’s seven-year civil war.

At worst, officials and experts have said, it could plunge both countries into bloody conflict. And at a minimum, squaring off in crowded battlefields has added to heightened tensions between Russia and the US as they each seek to exert influence in the Middle East.

Commanders of the rival militaries had long steered clear of the other by speaking through often-used deconfliction telephone lines. In the days leading up to the attack, and on opposite sides of the Euphrates River, Russia and the US were backing separate offensives against the Isis in Syria’s oil-rich Deir el-Zour province, which borders Iraq.

US military officials repeatedly warned about the growing mass of troops. But Russian military officials said they had no control over the fighters assembling near the river – even though US surveillance equipment monitoring radio transmissions had revealed the ground force was speaking in Russian.

The documents described the fighters as a “pro-regime force,” loyal to President Bashar al-Assad of Syria. It included some Syrian government soldiers and militias, but US military and intelligence officials have said a majority were private Russian paramilitary mercenaries – and most likely a part of the Wagner Group, a company often used by the Kremlin to carry out objectives that officials do not want to be connected to the Russian government.