A former Navy pilot said the memorandum of understanding reached Tuesday between Russia and the U.S. that dictates planes remain a "safe distance" from each other only means neither side wants to interfere in each others' operations.

"It is not definable, it is not enforceable, but it does show that neither side is trying to escalate the conflict," said Chris Harmer, a senior naval analyst with the Middle East Security Project at the Institute for the Study of War.

Russia and the U.S. have been sharing airspace over Syria for weeks as American jets strike Islamic State targets and the Russians strike rebels fighting against Syrian President Bashar Assad. Though in most cases pilots have behaved professionally, a couple close calls have occurred since Russia began bombing runs in late September, including a recent incident where aircraft from the two countries were just 500 feet apart, according to a CNN report.

The two countries signed a memorandum of understanding on Tuesday to keep pilots from both countries safe. The document requires professional airmanship, the use of specific communication frequencies and the establishment of a back-up communications line on the ground.

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Asked repeatedly about what constitutes a "safe distance" between planes, Pentagon press secretary Peter Cook declined to provide any numbers, estimates or even say if there are specific numbers spelled out in the agreement with the Russians.

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