WASHINGTON — The United States said Aug. 16 it was still assessing Russia’s surprise use of an Iranian air base to fly long-range bombers to strike targets in Syria — the first known time Iran has overtly allowed a foreign nation to use its military base since before the 1979 revolution. But while the State Department called the latest developments unhelpful, it said they would not derail ongoing US-Russian negotiations to restore a cease-fire to Syria and get political talks back on track.

“I think we are still trying to assess what exactly they are doing,” State Department spokesman Mark Toner told journalists at the department press briefing Aug. 16, referring to the Russians’ use of Iran’s Hamedan air base. “It is unfortunate, but not unexpected or surprising.”

Toner said, “Frankly, that only makes more difficult what is already a very contentious and complex situation, and pushes us farther away from what we are all trying to pursue: a credible nationwide cessation of hostilities and a political process in Geneva that leads to a peaceful transition.”

“What is unclear to us right now is the extent to which they are using Iran’s [base],” Toner added. “It may be a one-off thing. … We just don’t know at this point.”