The frequency of U.S. airstrikes on Islamic State targets in Syria has slowed significantly since the Russians began military operations in the region in early September, Defense Department data shows.

U.S. strikes averaged about seven per day in August, a pace similar to the entire first year of the air campaign in Syria that began last year, DoD data show.

But in September, after the Russians began setting up a military base at Syria's Latakia airport, the pace of U.S. airstrikes fell to an average of slightly more than four per day.

In October, as the Russians began airstrikes on targets in Syria supporting embattled President Bashar al-Assad, the average number of U.S. and coalition airstrikes fell slightly further, according to DoD data from Oct. 1 through Oct. 26.

But U.S. military officials say the drop in airstrikes is not related to the Russian military activity in the area.

"The Russian intervention has not significantly altered the Coalition's targeting in Syria," according to a statement Monday from the Combined Joint Task Force Operation Inherent Resolve.

"We strike targets where we find them and vet them and in the order we want to attack them. So there may ebbs and flows in the airstrike numbers," according to the statement, provided in response to an inquiry from Military Times.

In some cases, the Russians also are bombing Islamic State targets where the militants are threatening the regime of Moscow's ally, al-Assad. Yet U.S. officials say that is a small fraction of the total Russian strikes, which are mostly targeting other rebel groups threatening Assad.

For several weeks, the U.S. and Russia had no operational lines of communication as both militaries flew combat sorties in the same air space, in some cases coming within visual range, officials said.

But on Oct. 20, the U.S. and Russia forged a "memorandum of understanding" that outlined a series of safety procedures to prevent mishaps or misunderstandings.

Since the agreement was signed, U.S. and coalition airstrikes in Syria have slowed further. There was only one strike from Oct. 22 through Oct. 25, a lull not seen for months.

A Pentagon official said the unusually slow pace of airstrikes in late October was unrelated to the agreement with the Russians.

"We simply haven't had any [strikes] that we've done in the past few days. It doesn't mean we're not on the lookout for them," Navy Capt. Jeff Davis, a Defense Department spokesman, said Monday.

Some critics are unconvinced, and the Pentagon is facing criticism for the airspace deal with the Russians.

"This agreement means the United States is now moving out of the way and watching as Russian aircraft, together with Iranian, Hezbollah and Assad's ground forces, attack and kill brave Syrians," Sen. John McCain, R-Ariz., said at a Senate Armed Services Committee on Tuesday.

Defense Secretary Ash Carter, who testified before the committee, vowed that the Russian presence has not and will not make the U.S. military curtail its operations against the Islamic State, also known as ISIS and ISIL.

"To be clear, we are not cooperating with Russia and we're not letting Russia impact the pace or scope of our campaign against ISIL in Iraq and Syria," Carter said. "We will keep prosecuting our counter ISIL campaign unabated. We will keep supporting the moderate Syrian opposition, along with our other commitments to friends and allies in the region."

Carter did not directly acknowledge the slowdown in airstrikes, and in fact told lawmakers that the military has plans "to intensify our air campaign ... to target ISIL with a higher and heavier rate of strikes. This will include more strikes against ISIL high-value targets as our intelligence improves."

For now, overlap between U.S. and Russian areas of operation in Syria is limited, said Marine Gen. Joseph Dunford, chairman of the Joint Chiefs, who testified at the hearing alongside Carter.

"We are not operating in the same area as the Russians right now," Dunford said. "We've had two or three incidents where we had contact with Russian aircraft. ...That was before the signing of the memorandum of understanding."

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