Satellite imagery appearing to show the U.S. expanding a formerly disused air strip in Kurdish-controlled northern Syria has been seen by the BBC, the news agency reports.

The images, from the security analysts Stratfor, show a runway near the town of Rmeilan being extended from 700m to 1.3km.

That would make it more suitable for a larger aircraft such as a Hercules.

A spokesman for the U.S. Department of Defense said its small team in Syria needed "occasional logistical support."

Kurdish and other rebel forces are fighting against the Islamic State in that part of Syria. Rumors of U.S. military activity in the area have been circulating for weeks, Stratfor says.

Rmeilan is in Hassakeh province in north-east Syria. In October the U.S. dropped supply pallets containing more than 45 tonnes of ammunition to rebels in the province.