The State Department said Monday that it's not sure if Russia is done using an Iranian air base from which to launch attacks in Syria, a move that increased the complexity of the U.S. effort to fight the Islamic State in that country.

It was reported last week that Russia was using an Iranian air base, and the State Department said then that it was examining whether that was a violation of a United Nations resolution.

But on Monday, after reports that Russia stopped using the air base, the State Department said it wasn't quite sure what was going on.

"I'd have to refer you, frankly, to the governments of Russia and Iran to speak to what happened," said spokesman Mark Toner.

"We're monitoring closely, we continue to. It's not clear to us other than what we've seen in various press and public statements whether their use of this air base has definitively stopped, but we'll continue to watch it closely."

Toner did say that the Russia-Iran cooperation was part of the "alarming" picture in Syria, but he said he had no information about how their relationship may have changed.

"I'm not aware that we have information one way or the other what's going on here," he said.