Satellite images appearing to show the US expanding a formerly disused air strip in Kurdish-controlled northern Syria have been seen by the BBC.
The image, from the security analysts Stratfor, shows a runway near the town of Rmeilan being extended from 700m (half a mile) to 1.3km.
That would be long enough to land a Hercules aircraft.
BBC Arabic security correspondent Murad Shishani says it could be a new US move against the so-called Islamic State.
Rumours of US military activity in the area have been circulating for weeks, Stratfor says.
Rmeilan is in Hassakeh province in north-east Syria.
A US-led coalition of 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 airbase at Latakia, in western Syria.