(CNN) US military aircraft flying over Syria in the last few weeks have been subjected to unsafe flying practices by Russian aircraft on several occasions, according to a senior defense official.

"It's been sportier than we would like," the official said. "It's been less safe than we would like. From a flying point view, it has not been safe."

The situation has gotten to the point that US pilots have been "alerted" to the unsafe situation the official said. "They are on alert to be ready for unpredictable flying, and we are working with the Russians to figure out how to guarantee safer flying conditions." The official said some of the incidents also involve the flying practices of Syrian regime aircraft. "We have noticed the safety conditions are not where we want them to be."

American commanders and pilots are making certain to share everything they can with other crews in both safety and pre-flight briefings.

The US crews flying over the Middle Euphrates River Valley and Deir Ezzor are the most impacted by the unsafe Russian flights. Both militaries are operating in close proximity there against ISIS targets. The longstanding "deconfliction" communications channel between Russia and the US remains open. It was set up specifically to try to ensure there are no accidents or escalation in the use of forces.

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