The United States has seen a "limited" number of Russian military personnel, more akin to an exploratory party, looking at an airfield in Qamishli, Syria, possibly to determine how they might use the site, the officials said Thursday.
"We don't know what their intent is, but it is something we are watching closely," one official said, citing the already fraught relationship between Turkey and Russia after Turkey's shootdown last year of a Russian military jet that Ankara said had breached Turkish airspace.
Relations between the two nations, already at a low point, could be exacerbated if Russia built up its presence along the border, officials say.
One official called the development "very curious."
The airfield, formerly controlled by the regime of Syrian President Bashar al-Assad, also sits near the Kurdish-controlled region of northeastern Syria, where U.S. and coalition forces are active in the battle against ISIS, and far from Russia's established base of operations at an air base in Latakia along Syria's Mediterranean coast.
Officials say they are watching the developments very closely to see how the situation progresses, but are not yet at a stage of extreme concern.
Images may show U.S. activity
Meanwhile, satellite images published Friday by Stratfor show the apparent extension of a runway at an airfield in Rmeilan in al-Hasaka province, in the Kurdish-controlled region.
The private U.S. strategic analysis company said the images suggest the United States is expanding its support for Syrian rebels, though there was no confirmation the apparent extension is the work of the United States.
A spokesman for the U.S. Central Command denied the United States has taken control of any airfields in Syria.
"Our location and troop strength remains small and in keeping with what was previously briefed by the secretary of defense," the spokesman said.
"That said, U.S. forces in Syria are consistently looking for ways to increase efficiency for logistics and personnel recovery support, but there has been no change to the size or mission."
Separately, the London-based Syrian Observatory for Human Rights, citing "several reliable sources," said the Rmeilan airport would soon be ready for use by large American aircraft after expansion work in recent weeks.