By Phil Stewart and Yeganeh Torbati

WASHINGTON (Reuters) - For Pentagon officers who cut their teeth during the Cold War, the prospect of U.S. battlefield cooperation with Russia in Syria is not only uncomfortable. It's also unprecedented.

Against that background, the reactions of U.S. military officials range from caution to outright skepticism over a Geneva-based "joint integration center" that may soon bring together American and Russian militaries to discuss shared targets for the first time since World War Two.

"There are challenges with this. There is a trust deficit with the Russians," acknowledged General Joseph Votel, head of the U.S. military's Central Command, even as he voiced support for the initiative at a forum on Wednesday.

U.S. officials past and present voiced concerns about the initiative, underscoring the Pentagon's long-public criticism about the way Russia had been waging war in support of Syrian President Bashar al-Assad and over Moscow's 2014 annexation of Crimea from Ukraine.

Evelyn Farkas, a former deputy assistant secretary of defense who specialized in Russia, warned of dangers ahead.

"Conducting joint operations with the Russian military is fraught with political, military and potentially legal risk," Farkas told Reuters.

Under the deal, the United States and Russia are aiming for reduced violence over seven consecutive days before they move to the next stage of coordinating military strikes against Nusra Front and Islamic State militants, which are not party to the truce. If the truce holds, coordination could even start on Monday.

At that point, Russia and the United States could, in theory, gradually begin using the joint integration center to share targeting information.

Officials stress the Geneva-based JIC would not be similar to JOCs, the joint operation centers typical in war zones, like Iraq, replete with classified computer systems and giant television screens that show live feeds from armed drones carrying out strikes.

U.S. intelligence officials also have voiced concerns about sharing precise information on the positions of U.S.-backed rebel forces, given that Russia has targeted them in the past.

"The Russians aren't using precision-guided munitions in Syria, which gives them a perfect excuse to say, 'Sorry, we weren't aiming at your guys'," said one U.S. official, speaking on the condition of anonymity.


Working with Russia on targeting could risk linking Washington to any Russian misconduct.

Other U.S. officials publicly sought to play down those concerns this week, with one Obama administration official saying: "While we may share information on that threat, Russia remains fully responsible for the conduct of its operations."

There also is a legal hurdle. U.S. officials say Defense Secretary Ash Carter would need to issue a waiver to a U.S. law that puts strict limitations on U.S. military cooperation with Russia.

Carter, a fierce critic of Moscow, was skeptical of military coordination with Russia during internal Obama administration discussions. He has publicly backed the agreement and said on Wednesday the ceasefire, if implemented, would ease suffering.

"We in the Defense Department will play whatever role we have (in the process) with our accustomed excellence," he said.

Advocates for the initiative say the international community has run out of good choices in Syria's war, in which more than 400,000 people have died and more than 11 million people have been displaced.

"This whole war is the search for the least bad options," sad Anthony Cordesman at the Center for Strategic and International Studies in Washington.

Retired Admiral James Stavridis, a former supreme commander of NATO, supported the effort, even though he was not optimistic.

"The odds are low of this working out, given competing if not opposing (agendas). But it is worth a try given the dire humanitarian situation," Stavridis told Reuters.

(Additional reporting by Lesley Wroughton and John Walcott; Writing by Phil Stewart; Editing by Yara Bayoumy and Howard Goller)