Hundreds of Iranian ground troops have reportedly arrived in Syria over the past 10 days to join an offensive supporting embattled Syrian President Bashar al-Assad, two anonymous Lebanese sources told Reuters Thursday.

"The vanguard of Iranian ground forces began arriving in Syria: soldiers and officers specifically to participate in this battle," one of the unnamed sources told Reuters. "They are not advisors… we mean hundreds with equipment and weapons. They will be followed by more."

The sources added the offensive, in which Russia and other allied groups are also participating, is meant to reclaim territory that was captured from Assad by rebels in the Idlib and Hama provinces.

This claim of Iranian ground troops comes a day after Russian warplanes began striking anti-Assad rebel locations in the west of Syria near Homs and Hama. The involvement of Russia, a longtime ally of the Assad regime, has further escalated Syria's four-year civil war into a proxy battle with an ever-increasing number of players.

Iranian military advisors have been in Syria for a long time, points out Aron Lund, editor of the Carnegie Endowment for International Peace's Syria in Crisis. But they have been acting mostly as military advisors embedded with various militias.

"Putting Iranian soldiers on the ground in an independent frontline role would represent a pretty significant escalation, if that were to happen," Lund said. "Assad is short on manpower and long on enemies, and he needs all the help he can get."

Patrick Skinner, an analyst with the security consulting firm Soufan Group, agreed that the arrival of Iranian fighters, if true, is significant because it could signal the start of a much longer battle to retake rebel-controlled areas on the western coast of Syria. But he cautioned against taking the claims at face value in a conflict where everyone is battling for control of the narrative.

"This is an information war," Skinner said. "All the regional proxies — like Saudi Arabia, Iran, Russia — are going to start smearing everyone else. Everyone is trying to win the information war, plus win the actual war."

Hezbollah, a Lebanese militant group backed by Iran and loyal to Assad, has been involved in the fighting in Syria almost as long as the fighting has been taking place. Rebel Islamist groups are battling with pro-government forces in northwest Syria, and the Islamic State (IS) has taken wide swaths of the northeast of the country.

Meanwhile, Russia launched fresh strikes on rebel positions today, after denying accusations that their first wave of attacks killed 36 civilians yesterday. The US raised concerns that those being targeted by Russia were merely opponents of Assad, not IS militants.

"One of the horrors of this war is that it's so fluid and dynamic and every actor is seeking an advantage," Skinner said. "The only constant is the suffering of the Syrian people."