By Laila Bassam and Andrew Osborn

BEIRUT/MOSCOW (Reuters) - Hundreds of Iranian troops have 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 U.S. 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'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 mobilized 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 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 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." During the talks, Elissa Slotkin, an acting assistant U.S. secretary of defense, "noted U.S. concern that areas targeted by Russia so far were not ISIL strongholds." Cook said, using an acronym for Islamic State.

The Pentagon said it would not share U.S. intelligence with Russia and suggested the talks included ideas to increase safety, such as agreeing on radio frequencies for distress calls and a common language for communications.

U.S. Republican Senator John McCain, chairman of the Senate Armed Services Committee, and a frequent Obama critic, questioned the logic of talks on how to keep U.S. and Russian militaries apart, known in military parlance as "deconfliction."

“Unfortunately, it appears ‘deconfliction’ is merely an Orwellian euphemism for this administration’s acceptance of Russia’s expanded role in Syria, and as a consequence, for Assad’s continued brutalization of the Syrian people,” McCain said.

SAME ENEMIES, DIFFERENT FRIENDS

Russia's decision to join the war with air strikes on behalf of Assad, as well as the increased military involvement of Iran, could mark a turning point in a conflict that has drawn in most of the world's military powers.

Story continues