WASHINGTON (AP) — The Pentagon is grappling with whether the U.S. should use military force to protect U.S. trained and equipped Syrian rebels now that they may be the targets of Russian airstrikes.

Senior military leaders and defense officials are working through the thorny legal and foreign policy issues and are weighing the risks of using force in response to a Russian attack, U.S. officials said Thursday.

Pentagon leaders have consistently said that the U.S. must take steps to protect the American-trained rebels because it would be far more difficult to recruit fighters without those assurances. Defense Secretary Ash Carter told reporters in March that the U.S. has an obligation to support them, “And we’re working through what kinds of support and under what conditions we would do so.”

U.S. officials later made it clear that rebels trained by the U.S. would receive air support in the event they are attacked by either Islamic State militants or Syrian government troops. Currently, that protection would apply only to about 80 U.S.-trained Syrian rebels who are back in Syria fighting with their units.

The U.S. policy so far is very specific. It doesn’t address a potential attack by Russian planes and does not include Syrian rebels who have not been through the U.S. military training, even though they may be aligned with the U.S. or IS.

Carter declined to discuss the problem when asked about it this week. But U.S. officials acknowledged that this is one of the questions being asked as they debate the administration’s response to what White House press secretary Josh Earnest described as Russia’s “indiscriminate military operations against the Syrian opposition.”

A key concern is the prospect of U.S. getting drawn into a proxy war with Russia in the event that Russian warplanes hit moderate Syrian rebels who have been trained and equipped by the U.S. military.

The officials spoke on condition of anonymity because they were not authorized to discuss ongoing deliberations publicly.

Tensions between the U.S. and Russia are escalating over Russian airstrikes that apparently are serving to strengthen Syrian President Bashar Assad by targeting rebels — perhaps including some aligned with the U.S. — rather than hitting Islamic State fighters it promised to attack.

The U.S. and its allies have been calling on Russia to immediately cease attacks on the Syrian opposition and to focus on fighting Islamic State militants, saying Moscow’s actions will “only fuel more extremism and radicalization.”

The Pentagon on Thursday had its first conversation with Russian officials in an effort to avoid any unintended U.S.-Russian confrontations as the airstrikes continue in the skies over Syria. During the video call, Elissa Slotkin, who represented the U.S., expressed America’s concerns that Russia is targeting areas where there are few if any Islamic State forces operating. Slotkin is the acting assistant secretary of defense for international security affairs.

The U.S. side proposed using specific international radio frequencies for distress calls by military pilots flying in Syrian airspace, but he was not more specific about that or other proposals.

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Associated Press writers Josh Lederman and Matthew Lee contributed to this report.

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