Beirut, Moscow: Hundreds of Iranian troops have arrived in Syria to join a major ground offensive on behalf of President Bashar Al Assad’s government, Lebanese sources said on Thursday, a further step in the rapid internationalisation of a civil war in which every major country in the region has a stake.

Russian warplanes that joined the fight this week bombed a camp run by rebels trained by the CIA, 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.

The US and Russian militaries were to hold talks via video link to seek ways to keep their militaries apart as they wage parallel campaigns of air strikes in Syria, a US defence official said.

Russian jets struck targets near the cities of Hama and Homs in western Syria on the second day of their surprise air campaign.

Moscow said it had hit Daesh positions, but the areas it struck are mostly held by a rival insurgent alliance, which is supported by US allies including Arab states and Turkey.

Hassan Haj Ali, head of the Liwa Suqour Al Jabal rebel group, said one of the targets was his group’s base in Idlib province, struck by around 20 missiles in two separate sorties. His fighters had been trained by the CIA in Qatar and Saudi Arabia, part of a programme Washington says is aimed at supporting groups that oppose both Daesh and Al Assad.

“Russia is challenging everyone and saying there is no alternative to Bashar,” Haj Ali said. He said the Russian jets had been identified by members of his group who once served as Syrian air force pilots.

Two Lebanese sources said 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 Al Assad’s Lebanese Hezbollah allies and by Shiite militia fighters from Iraq, while the Russians would provide air support.

“The vanguard of Iranian ground forces began arriving in Syria: soldiers and officers specifically to participate in this battle. They are not advisers ... we mean hundreds with equipment and weapons. They will be followed by more,” one of the sources said.