WASHINGTON—The U.S. asked Moscow to force the regime of President Bashar al-Assad to ground its air force as part of a proposal that could lead to greater cooperation with Russia in Syria, according to officials familiar with the offer.

Advocates of the proposal say it could help put an end to the regime's barrel-bomb attacks on opposition enclaves, a longstanding Obama administration objective.

If the Russians get the regime to ground its air force, the Pentagon then would consider helping Moscow with its targeting in Syria, to ensure its strikes hit the Nusra Front and Islamic State militant groups, rather than the U.S.-backed moderate opposition, the officials said.

The U.S. has delivered the proposal to the Russians, according to a U.S. official. But this official and others said it was unlikely to yield a result or a substantive response for many days or even weeks. "I don't think it's a foregone conclusion that the Russians want it," a U.S. official said.

Russian officials in Washington didn't respond to a request to comment.

Details about the proposal were earlier reported Thursday by the Washington Post.

A senior U.S. official said that the Russians would have to get the Assad regime to ground its air force first. That would prompt the U.S. side to consider some degree of coordination with Moscow in Syria.

But the prospect of passing specific targets on to the Russians is fraught, the official said, because the U.S. would still feel morally responsible for the results of any strike. For that reason, the official said, it would be very cautious about sharing that information.

Defense Secretary Ash Carter has long been wary of any coordination with Russia in Syria, and it isn't clear to what degree the U.S. would begin to provide any detailed targeting information to Moscow, the official said.

At a Pentagon news conference, Mr. Carter, without speaking directly to the proposal at hand, said that the U.S. has long been willing to work with the Russians if they would keep their promises to go after Islamic State and not the moderate opposition.

"If the Russians would do the right thing in Syria, and that's an important condition, as in all cases with Russia, we're willing to work with them," Mr. Carter said at the Pentagon Thursday.

John Kirby, the State Department spokesman, declined to discuss "the details of any ongoing internal or diplomatic conversations."

"We have been clear about Russia's obligations to ensure regime compliance with the cessation of hostilities. We have also been clear about the danger posed by al Qaeda in Syria to our own national security," Mr. Kirby said in a statement. "We are looking at a number of measures to address both of these issues."

Greater cooperation would likely happen in careful sequence, based on a quid-pro-quo relationship with the Russians in which they would demonstrate their commitment to it, the U.S. official said.

"If you get the Russians to ground the air force, that's a big deal," the official said.

By providing information to Russian forces about what the U.S. considers to be legitimate terrorist targets, U.S. officials hope to steer Russian aircraft away from striking the U.S.-backed moderate opposition.

Since its intervention in Syria last year, Moscow has lumped Central Intelligence Agency-backed rebels in with the Nusra Front, branding them all "terrorists" because they are fighting the Assad regime.

Administration officials say the proposed deal, if it advances, wouldn't change President Barack Obama's position that Mr. Assad needs to leave office.

Critics within the administration say, however, that the proposal amounts to a concession by Mr. Obama on that position since it would give Russia the kind of legitimacy it is seeking in the fight in Syria without compelling Moscow to push Mr. Assad aside.

(END) Dow Jones Newswires

June 30, 2016 21:05 ET (01:05 GMT)