The US was warned yesterday it is facing "a train wreck" in Syria amid an escalating confrontation between Turkey and Kurdish forces over who should lead the final assault on the Isil stronghold of Raqqa.
The Pentagon began laying the foundation for the siege this week by deploying several hundred marines to northern Syria armed with heavy artillery. But the assault on Raqqa threatened to descend into chaos after the Turkish government and the main US-backed rebels in Syria each demanded that America exclude the other from taking part in the attack.
Turkey's prime minister accused Kurdish fighters within the Syrian Democratic Forces (SDF) rebel group of being "terrorists", while the SDF said Turkish troops inside Syria were an "occupation force" who must leave.
The tension has escalated to the point that the US has deployed its own troops between the two sides in an effort to prevent confrontation between them, and a senior American general conceded he was worried the rivalry could undermine the fight against Isil.
"I foresee a train wreck here," John McCain, the senior Republican senator, told General Joseph Votel, the commander of US forces in the Middle East, at a hearing in Washington.
The general said he was also concerned and added: "We are trying to take actions to prevent [a clash] from occurring."
The showdown between Turkey and the SDF highlights the difficulties facing Donald Trump as he takes over the complex task of managing rival US allies in Syria and rallying them for the attack on Raqqa.
The US artillery units that began arriving this week will be tasked with pounding Isil's defences as Syrian rebel forces cut off its supply lines and surround the city in anticipation of a full assault. The marines are only deployed temporarily but the move signals Mr Trump's willingness to send more conventional ground troops into Syria and to give more freedom to military commanders to make decisions on troop levels. The approach contrasts with the Obama administration, where the White House imposed strict limits on US numbers in Syria and preferred sending clusters of special forces over large numbers of traditional troops.
Mr Trump's advisers are also weighing a plan to send a reserve force of 1,000 troops to Kuwait and giving commanders the flexibility to deploy them to either Iraq or Syria. Gen Votel also suggested more US troops might be needed in Syria to ensure stability once Isil was defeated.
The additional movements around Raqqa came as bitter fighting continued in Mosul, where US-backed Iraqi forces are driving Isil from its final city stronghold in Iraq. The jihadist group took responsibility for an attack carried out by two suicide bombers who struck a wedding in a village near the Iraqi city of Tikrit, killing 26 people. The complexity in northern Syria is only added to by the presence of forces from the Syrian regime and the Russian military. Mr Trump has not yet decided on how closely the US should co-operate with either the Russians or the Assad regime but has in the past appeared more open to co-operation than Barack Obama.
Russian troops have been seen in northern Syria and the close proximity raises questions about what role the Syrian regime might play in the attack on Raqqa. Vladimir Evseev, a Russian military expert at the Commonwealth of Independent States Institution, said it was possible that the US and Assad regime would directly collaborate, marking a turning point in US policy that has so far shunned collaboration with Damascus.
Meanwhile, air strikes killed 23 civilians, including eight children, yesterday in countryside around Raqqa, by warplanes understood to belong to the US-led coalition.