Defense Secretary Ashton Carter on Tuesday told the Pentagon to communicate with Russia about coalition activity in Syria, in a bid to avoid accidents as Moscow builds a military presence in the war-torn country.
The Pentagon says Russia has in recent weeks sent bombers, fighter jets, at least 500 troops and a slew of other military hardware to the Latakia region in northwestern Syria.
Defense officials have stressed the need for "deconfliction" of coalition and Russian forces, meaning communicating about whose aircraft are where to reduce the risk of accidents.
"This morning, Secretary Carter directed his staff to open lines of communication with Russia on deconfliction," Pentagon press secretary Peter Cook told reporters.
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"The safety of coalition pilots (is) critically important to us. We do not want misjudgment and miscalculation. We do not want an accident to take place," he added.
Russia and the United States have diverging views on how to tackle the Islamic State group in Syria.
The United States leads a coalition that conducts regular strikes against jihadists in Iraq and Syria, and President Barack Obama is against any support for Syrian President Bashar al-Assad, a longtime ally of Moscow.
But Russian President Vladimir Putin on Monday called for a broad coalition to fight IS that would include Syria's army, and observers expect any Russian military action in Syria would be in support of Assad.
The 4.5-year war has killed more than 240,000 people and displaced millions more.