They told AFP work was underway to expand an airfield in Rmeilan, in Hasakeh province, from where aircraft used to take off to spray pesticides on crops before Syria's war started five years ago.
A Syrian military source said nearly 100 "American experts," alongside forces from the anti-IS Kurdish People's Protection Units had widened the landing strip and refurbished some infrastructure.
"The airbase will be used for helicopters and cargo planes. Its strip is now 2,700 metres (yards) long and is ready to be used by planes that will transport equipment and ammunition," the source added.
The Pentagon did not confirm the reports but said the US military had not taken control of any airfield in Syria.
"There has been no change to the size of mission of the US presence in Syria," added US Central Command spokesman Colonel Pat Ryder.
"That being said, US forces in Syria are consistently looking at ways to increase efficiency for logistics and personnel recovery support," he said.
Signup to our newsletter and follow us on Facebook and Twitter!
At the end of October, US President Barack Obama authorised the deployment of up to 50 special operations troops to Syria to advise local ground forces battling IS.
The military source told AFP that US forces have been working on the Rmeilan airfield "for more than three months."
A security source in northeast Syria said "American special forces and advisors are using the Rmeilan airport as a base, from where helicopters are taking off towards the fronts."
The Britain-based Syrian Observatory for Human Rights said the airstrip has been widened and "is nearly ready for us by American planes".
The US is supporting a Kurdish-Arab alliance called the Syrian Democratic Forces against IS in northern Syria, providing it with air cover as part of a broad coalition battling IS.
SDF spokesman Talal Sello denied that US forces had taken over the Rmeilan, describing it as "an agricultural airport."
The US-led coalition began bombing IS targets in Syria in September 2014, expanding their mission from neighbouring Iraq.