Russian warplanes, in a second day of strikes, 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.
Speaking by video link for an hour, U.S. and Russian military officials discussed ways to ensure their warplanes do not come into conflict as they carry out separate air campaigns over Syria, White House spokesman Josh Earnest told reporters. He said it was the first in a series of conversations.
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'ite militia fighters from Iraq, while Russia 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.
So far, direct Iranian military support for Assad has come mostly in the form of military advisers. Iran has also mobilised Shi'ite militia fighters, including Iraqis and some Afghans, to fight alongside Syrian government forces.
Moscow said it had hit Islamic State 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 which is part of the Free Syrian Army, told Reuters one of the targets was his group's base in Idlib province, struck by around 20 missiles in two separate raids. 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 Islamic State and 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.
The group is one of at least three foreign-backed FSA rebel factions to say they had been hit by the Russians in the last two days.
At the United Nations, Russian Foreign Minister Sergei Lavrov told a news conference Moscow was targeting Islamic State. He did not specifically deny that Russian planes had attacked Free Syrian Army facilities but said Russia did not view it as a terrorist group and viewed it as part of a political solution in Syria.
The aim is to help the Syrian armed forces "in their weak spots", said Kremlin spokesman Dmitry Peskov.
Pentagon spokesman Peter Cook described Thursday's military talks as "cordial and professional" and said a U.S. official raised concerns that areas targeted by Russian aircraft in Syria were not Islamic State strongholds.
German Foreign Minister Frank-Walter Steinmeier told the United Nations on Thursday: "Instead of lone decisions by Russia to take direct military action in Syria we need Russia to take political action advocating transition in Syria."