Satellite imagery appearing to show the US expanding a formerly disused air strip in Kurdish-controlled northern Syria has been seen by the BBC.
The images, from the security analysts Stratfor, show a runway near the town of Rmeilan being extended from 700m (half a mile) to 1.3km.
That would make it more suitable for a larger aircraft such as a Hercules.
A spokesman for the US Department of Defence said its small team in Syria needed "occasional logistical support".
Kurdish and other rebel forces are fighting against the so-called Islamic State in that part of Syria.
Rumours of US military activity in the area have been circulating for weeks, Stratfor says.
Rmeilan is in Hassakeh province in north-east Syria. In October the US dropped supply pallets containing more than 45 tonnes of ammunition to rebels in the province.
Murad Batal Shishani, BBC Islamic Groups analyst
The imagery appears to confirm growing rumours over the past couple of days about the presence of a runway.
This development could signify a new approach by the Americans in support of their Kurdish allies in the war against the so-called Islamic State.
But it could trigger an angry reaction from major US ally Turkey - which has long been worried about Kurdish ambitions on its southern border.
Also, the runway's closeness to Iraq's second largest city, Mosul - currently under IS control - suggests that efforts to retake the city are at stake. It seems Kurds will be on the front lines, fighting for their historical ambition of a Kurdish state.
Access to a longer runway would facilitate US deliveries of weapons to rebel forces in the area instead of depending on airdrops.
A US-led coalition of Western and Middle Eastern countries began air strikes against IS in Iraq in August 2014 and in Syria a month later.
Turkey has allowed the US to use its Incirlik airbase, in the south of the country, near north-west Syria.
Russia began carrying out its own air strikes in Syria in September 2015 after a request from Syrian President Bashar al-Assad.
He has clung on to power despite more than four years of civil war. Russia has an air base at Latakia, in western Syria.