Russian airstrike in Syria (archive)
Russian airstrike in Syria (archive)Reuters

The United States on Tuesday bemoaned Russia's use of an Iranian air base to launch a bombing raid in Syria, but credited Moscow for having given a brief advance warning, AFP reported.

Earlier on Tuesday, Russia's defense ministry said long-range bombers and fighter jets took off from the Hamedan base in western Iran and "conducted a group air strike against targets of the Islamic State (ISIS) and the Jabhat Al-Nusra terrorist groups" in the Syrian provinces of Aleppo, Deir Ezzor and Idlib.

"It's unfortunate, but not surprising or unexpected," State Department spokesman Mark Toner said.

"Frankly, that only makes more difficult what is already a very contentious and complex and difficult situation, he added, according to AFP.

"And it only pushes us further away from what we're all... trying to pursue, which is a credible nation-wide cessation of hostilities and a political process in Geneva that leads to a peaceful transition," continued Toner.

Earlier, Baghdad-based U.S. military spokesman Colonel Chris Garver said Russian authorities had notified the American-led coalition fighting ISIS in Iraq and Syria shortly before launching the bombing mission from Iran.

The coalition since last year has operated a "memorandum of understanding" with Russia, whereby the two military forces notify each other of flights during their separate bombing campaigns to avoid accidents in the skies over Syria.

"The Russians did notify the coalition as per the memorandum of understanding for safety of flight," Garver said.

"They informed us they were coming through and we ensured safety of flight as those bombers passed through the area and toward their target and then when they passed out again. They did not impact coalition operations in either Iraq or Syria during the time," he added.

Garver noted that ISIS fighters are concentrated only in Deir Ezzor and not Aleppo or Idlib.

Moscow, which began airstrikes in Syria last September, claims it is striking ISIS and other "terrorists", but Washington and its allies have in the past accused Russia of targeting moderate rebels as well.

Toner said on Tuesday that Russia continues to "predominantly target moderate Syrian opposition forces."

Moscow had provided "not a lot" of warning but that it "was enough time to make sure that we could ensure safety of flight," Garver said.

He did not comment when asked if Russia had sought overflight permission from the government of Iraq, whose airspace provides quickest access to Syria from Iran.

Russia on Monday said it and the United States were close to joining forces in some form around Aleppo, but U.S. officials have not confirmed this.

"We continue to speak with Russia... about ways that we can put in place a credible, nationwide cease-fire, full access to humanitarian assistance, and then again get negotiations restarted in Geneva," Toner said.

AFP contributed to this report.