"The recent incident once again shows that the United States' illegal military presence in Syria is actually aimed at taking control of the country's economic assets and not at fighting against the ISIS international terror group," said a statement issued by the Russian Defence Ministry. There were no US casualties, and one SDF fighter was injured, the US military said. It was the most serious clash involving the United States and Syria since US troops began deploying to northeastern Syria in late 2015 in support of Kurdish and Arab fighters against the Islamic State. It illustrated the complexity of the battlefield now that the Islamic State war is winding down, leaving the United States in overall control of roughly a quarter of Syria. A deconfliction line establishing the Euphrates River as the divide between the United States and its SDF allies and Russia and its Syrian government allies was negotiated between the US and Russian militaries last year, and until now it has served to mostly keep the rival armies apart. The Syrian government and its ally Iran have repeatedly called for US troops to leave Syria now that the fight against the Islamic State is over, and have regularly threatened to wage war to push them out if they don't leave.

Beaten by the government and squeezed by al-Qaeda, many of Syria's rebels have put themselves under Turkey's wing to battle Kurdish forces, trying to find a path back to relevance in Syria's messy and multi-sided civil war. Credit:AP But the Trump administration announced a new Syria strategy last month that envisages US troops remaining in eastern Syria until there is a peace settlement to the wider war that includes a transition away from the rule of President Bashar Assad. With no sign of a peace settlement in sight, the US military is now committed to a potentially indefinite presence in Syria that is opposed by all the region's powers. The prospect of the United States remaining in Syria while shoring up Kurdish efforts to secure self-rule has provoked a convergence among those opposed to any form of Kurdish autonomy, uniting Turkey, Russia, Iran and the Syrian government in a de facto alliance against the US presence. The United States "is now in mission creep mode," said Robert Ford, a former US ambassador to Syria who is now a senior fellow at the Middle East Institute.

"The Americans have managed through their diplomatic strategy to isolate themselves to the point where Turkey, Iran and Syria all agree that what the US is doing in Syria is bad," he said. The attack on the southernmost flank of the SDF-controlled area in northeastern Syria came as Turkey pressed ahead with its nearly two-week old offensive against the Kurdish enclave of Afrin in northwestern Syria, which is controlled by the same Kurdish force that dominates the SDF but does not have direct US support. Turkey has also been threatening to attack the US-backed forces further east, around the town of Manbij, where US troops mount regular patrols. The attack in Deir al-Zour coincided with a visit by Lieutenant Gen Paul Funk, the commander of US forces in Iraq and Syria, to the frontlines in Manbij, where he offered assurances to the SDF of US support. The Kurdish led force is feeling increasingly "under attack from all sides," said Kino Gabriel, a spokesman for the SDF in northeastern Syria. "This is an attack on the political project we are trying to establish in Syria." If the tensions persist, the SDF and their US military advisers could soon find themselves fighting a war on three fronts, against Turkey, the Islamic State and the Syrian government alliance. But, Gabriel said, the SDF is confident the force has enough fighters both to confront the Turkish incursion and hold its lines in Deir al-Zour.

He said the attack on the SDF base was conducted by members of the National Defence Force, a Syrian militia that is loyal to the Syrian government and has received arms and supplies from Iran and the Lebanese Hezbollah movement. The likely goal, he said, was to attempt to halt any further SDF advances further east, where only a few Islamic State-held villages lie between SDF positions and the Iraqi border. "We assume it is because they want to divert our attention from the ongoing operations against Daesh in the Euphrates Valley," he said. "We only have a few more towns and villages to be cleared of ISIS, they are completely surrounded and we are just waiting to finish our operations and clear the Iraqi Syrian border." Daesh is the Arabic acronym for the Islamic State. The US military spokesman Veale said the hostilities erupted after several hundred infantry forces backed by tanks and artillery began advancing on an SDF position in the town of Khisham, located around 8 kilometers east of the deconfliction line. It lies near some of Syria's biggest oil fields that are currently under SDF control, and he speculated that the pro-government forces were trying to take them back. When the advancing forces unleashed a volley of 20-30 tank and artillery rounds toward the base, US warplanes and artillery struck back, he said in an emailed statement. "This action was taken in self-defence," he said. Channels of communication between the US and Russia remained open throughout the Thursday night clash, Veale said, and the US informed Russia before it opened fire on the pro-government force.

The Russian Defense Ministry statement however that said the battle occurred when a reconnaissance party made up of Syrian militias ventured across the Euphrates River to hunt down an Islamic State position, only to be attacked unprovoked by US warplanes. The militias had neglected to inform the Russian military of their plan in advance, the Russian statement added. Washington Post