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.
"There has been no change to the size of mission of the US presence in Syria," added US Central Command spokesman Colonel Pat Ryder.
More From This Section
"That being said, US forces in Syria are consistently looking at ways to increase efficiency for logistics and personnel recovery support," he said.
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."
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."