The U.S. military agreed to stay out of the way of Russian bombers as they flew through Iraqi airspace from newly established bases in Iran Tuesday to attack targets in Syria.

"The Russians did notify the coalition as per the memorandum of understanding for safety of flight," said U.S. military spokesman Col. Chris Garver. "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."

The Russian airstrikes hit at least two locations where the U.S. says there are no concentrations of Islamic State forces.

But Pentagon officials insisted the U.S. had no authority to stop the flights once a formal notification was made.

The Russian Defense Ministry said its bombers took off from the Hamadan airbase in northwest Iran and targeted both Islamic State fighters as well as militants previously known as the Nusra Front in three locations: Aleppo, Idlib and Deir el-Zour provinces.

Garver says there was no significant changes to coalition air operations in Iraq and Syria, which he said were unaffected by the Russian flights.

While not confirming the targets, Garver said in a briefing for reporters at the Pentagon from Baghdad that two areas the Russians claimed to have bombed had few if any fighters from the Islamic State, which the U.S. also refers to by its Arabic acronym "Daesh."

"ISIS is in Deir el-Zour. We have struck targets ourselves in Deir el-Zour," Garver said. "We have not struck targets in Aleppo in a very long time. We have not struck targets in Idlib in a very long time, if we have at all. We don't see concentrations of ISIS in those areas."

The Russian strikes come as reports from Moscow quote Russian Defense Minister Sergei Shoigu as saying Moscow and Washington are close to beginning joint military action against militants in Aleppo

A Russian Defense Ministry statement said the Tuesday airstrikes by Russian Tu-22 bombers destroyed five large arms depots, along with a militant training camp, three command-and-control points, and a significant number of militants."

While Garver stopped short of directly accusing the Russians of bombing rebels or anti-Islamic State forces supported by the U.S., the questions about who they are targeting underscore the deep reservations among many in the Pentagon about coordinating with the Russians in the fight against the Islamic State, namely they believe you can't trust them to do what they say they are going to do.

"We have seen Russians strike targets that we did not think were Daesh. I specifically said the three targets that they said that they went to hit in Idlib and in Aleppo and in Deir el-Zour, we see Daesh in Deir el-Zour," Garver said. "I didn't really think I needed to go any further than that to call that out."

Aleppo, once Syria's largest city, is on the front lines in the civil war. Rebels hold some areas, while government forces loyal to President Bashar Assad hold other areas.

The U.S. has consistently accused the Russians of claiming to attack Islamic State positions, when in fact they are backing the Assad regime forces against the rebels.

Humanitarian groups say hundreds of thousands of Syrian civilians remain trapped in rebel areas and are at risk of being isolated if Assad forces succeed in capturing the last corridor linking them with the outside.

The United States says Assad must step down, while Russia is backing Assad both politically and militarily.

CORRECTION: A previous version of this story incorrectly stated that the U.S. cleared the way for the Russian planes to fly through Iraqi airspace. The Pentagon said that under the safety of flight memorandum with the Russians, Russia doesn't need permission from the U.S. to conduct those flights.