Image copyright © 2019 Getty Images Image caption The Americans have continued to train and equip local forces in northern Syria
US officials say the effort to help the Syrian Democratic Forces (SDF) against Islamic State is another means to fight the group and help refugees.
Gen Patrick Shea said the US wanted to help the SDF to regain their sovereign capabilities, using a mix of active measures.
He said: “These are not our boots on the ground. This is an effort to work with the SDF to counter the threat and to develop their security and sovereignty.”
The US has been training and equipping the SDF for more than a year.
The Americans are among four main contributors to the 70,000-strong SDF, which has captured roughly one third of the territory controlled by the IS militant group in Syria.
Diplomats have told the BBC that Europe should not expect a quick return to the refugee camps it was forced to open in 2015 in response to a migrant crisis.
They say any repatriation process could take years to carry out.
Gen Shea, who led the US military’s efforts in Iraq and Afghanistan before his appointment, said the US was working to “help the SDF regain their sovereignty… help them to develop their security and sovereignty”, as well as help refugees.
Meanwhile, British Defence Secretary Gavin Williamson defended the UK’s contribution to the effort.
“This is a critical period for the region and we must take our share of responsibility. Britain did a better job in the beginning than we have done in recent months,” he said.
US General John Nicholson, the commander of the US-led coalition against IS, said the campaign was “advancing” against the militants, who still controlled large territory in the Euphrates valley near the Iraqi border.
He said these militants had “already degraded” and “will lose their last strongholds” in the coming months.
Gen Nicholson said the efforts were “not a Nato effort, it’s a broader effort” by the coalition.
He said: “I am fully confident that as we establish areas of joint responsibility, we have removed the core from Daesh (IS).”
Speaking to the BBC, he added: “We’ve achieved a very, very big result. The last forces of Daesh will be on the run soon. There will be no safe haven left.”
The US military leader said the fight against IS in Iraq and Syria had reached a decisive phase.
“We expect that will continue for the coming year and a half.”
He also said no plans for full-scale American military withdrawal were imminent.
The US says it is providing SDF with air, artillery and special forces support.
Under a partial troop withdrawal deal agreed in May, American forces and their allies are to pull back to about 4,000 troops, with the SDF moving in on their IS strongholds.