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, US officials say.
One US 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.
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"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.
Islamic State fight
The United States has long warned Syria not to interfere in US-led coalition air operations, which target Islamic State 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 Islamic State.
"We would welcome constructive contributions from the Russians to the anti-ISIL coalition," Earnest said, using an acronym for Islamic State.
"So that's why we'll remain open to tactical, practical discussions with the Russians in order to further the goals of the counter-ISIL coalition and to ensure the safe conduct of coalition operations."
US Secretary of State John Kerry said on Wednesday that Russia had sought military-to-military talks on Syria.
Kerry and US Defense Secretary Ash Carter met at the Pentagon on Thursday, with Syria believed to be high on the agenda. Kerry was later flying to London for talks, separately, with his British and United Arab Emirates counterparts.
The American war effort in Syria has seen recent setbacks. The US military acknowledged this week that only four or five of an initial class of some 60 US-trained Syrian rebels were still on the battlefield, following a disastrous debut after they came under attack from al Qaeda's Syria wing.
The training program could be re-envisioned as a far smaller training effort focused on more easily reachable goals, such as helping call in air strikes, US officials say.
Both Moscow and Washington say their enemy is Islamic State, whose fighters control large parts of Syria. Washington believes that Assad's presence is fueling the conflict, but Russia continues to support its long-time ally.
Washington has been pressing Moscow to help with a political transition in Syria, which would see Assad hand over power to an interim governing body. But Moscow's military buildup has complicated those efforts.
