DAMASCUS — Hundreds of Iranian troops arrived in Syria to join a major ground offensive in support of President Bashar al-Assad’s government, Lebanese sources said on Thursday, a sign the civil war is turning still more regional and global in scope.

Russian warplanes, in a second day of strikes, bombed a camp run by rebels trained by the Central Intelligence Agency, the group’s commander said, putting Moscow and Washington on opposing sides in a Middle East conflict for the first time since the Cold War.

Senior U.S. and Russian officials spoke for just over an hour by secure video conference on Thursday, focusing on ways to keep air crews safe, the Pentagon said, as the two militaries carry out parallel campaigns with competing objectives.

“We made crystal clear that, at a minimum, the priority here should be the safe operation of the air crews over Syria,” Pentagon spokesman Peter Cook said.

Two Lebanese sources told Reuters hundreds of Iranian troops had reached Syria in the past 10 days with weapons to mount a major ground offensive. They would also be backed by Assad’s Lebanese Hezbollah allies and by Shi’a militia fighters from Iraq, while Russia would provide air support.

So far, direct Iranian military support for Assad has come mostly in the form of military advisers. Iran has also mobilized Shi’a militia fighters, including Iraqis and some Afghans, to fight alongside Syrian government forces.

Moscow said it had hit ISIS positions, but the areas it struck near the cities of Hama and Homs are mostly held by a rival insurgent alliance, which unlike Islamic State is supported by U.S. allies, including Arab states and Turkey.

Hassan Haj Ali, head of the Liwa Suqour al-Jabal rebel group that is part of the Free Syrian Army, told Reuters one of the targets was his group’s base in Idlib province, struck by about 20 missiles in two separate raids. His fighters had been trained by the CIA in Qatar and Saudi Arabia, part of a program Washington says is aimed at supporting groups that oppose both ISIS and Assad.