Obama meets with Putin in Northern Ireland, June 17, 2013.

The United States opened the door on Thursday to possible tactical discussions with Russia about the war in Syria, as Moscow’s deepening military presence raised the prospect of some limited coordination between the former Cold War foes.

The White House said it “remains open to tactical, practical discussions” with Moscow. The Pentagon acknowledged that such talks might be necessary to avoid “miscalculation,” although it was unclear when or under what conditions they might take place.

The United States fiercely opposes Russia’s support for Syrian President Bashar al-Assad, and the Pentagon last year cut off high-level discussions with Moscow after its annexation of Crimea and intervention in Ukraine.

But Russia’s buildup at Syria’s Latakia airbase raises the possibility of air combat missions in Syrian airspace. Heavy Russian equipment, including tanks, helicopters and naval infantry forces, have been moved to Latakia, U.S. officials say.

One U.S. official told Reuters on Thursday that the United States believed as many as 500 Russian troops were now at the airfield, an increase from a previous estimate of about 200.

“We want to avoid miscalculation. We want to avoid problems,” Pentagon spokesman Peter Cook told a news briefing, noting however that Russia wasn’t carrying out such operations now and had not yet explained its intentions.

ISIS FIGHT

The United States has long warned Syria not to interfere in U.S.-led coalition air operations, which target ISIS militants in Syria, not Syria’s military.

At the White House, spokesman Josh Earnest raised the prospect of possible Russian contributions to the campaign against ISIS.

“We would welcome constructive contributions from the Russians to the anti-ISIL coalition,” Earnest said, using an acronym for ISIS.

U.S. Secretary of State John Kerry said on Wednesday that Russia had sought military-to-military talks on Syria.