"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 President 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 Isis' 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 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 that of the Obama Administration, which imposed strict limits on US numbers in Syria and preferred sending clusters of special forces over large numbers of traditional troops.
Trump's advisers are also weighing a plan to send a reserve force of 1000 troops to Kuwait and giving commanders the flexibility to deploy them to either Iraq or Syria.
Votel also suggested more US troops might be needed in Syria to ensure stability once Isis was defeated.
The additional movements around Raqqa came as bitter fighting continued in Mosul, where US-backed Iraqi forces are driving Isis from their 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.
Trump has not yet decided 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 was.
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 the Assad regime would directly collaborate - marking a turning point in US policy that has so far shunned collaboration with Damascus.
Meanwhile, airstrikes killed 23 civilians, including eight children, yesterday in countryside around Raqqa, by warplanes understood to belong to the US-led coalition, the British-based Syrian Observatory for Human Rights said.
