The development is almost certain to increase pressure on Western-backed rebels, who are battling multiple foes, and push more civilians out of the areas of fighting, potentially creating a fresh wave of refugees.

Russia began its air campaign Sept. 30, and Syrian troops and allied militiamen launched a ground offensive against rebels in central Syria a week later. Russia says its airstrikes are meant to weaken the ISIS and other “terrorists” in Syria, but Western officials and Syrian rebels say most of the strikes have focused on central and northern Syria, where the extremist group does not have a strong presence.

The official, who has deep knowledge of operational details in Syria, said the Iranian Revolutionary Guards - currently numbering around 1,500 - began arriving about two weeks ago, after the Russian airstrikes began, and have accelerated recently. The Iranian-backed group Hezbollah has also sent a fresh wave of fighters to Syria, he told The Associated Press.

Iranian and Syrian officials have long acknowledged Iran has advisers and military experts in Syria, but denied there were any ground troops. Wednesday’s statements were the first confirmation of Iranian fighters taking part in combat operations in Syria.

The main goal is reportedly to secure the strategic Hama-Aleppo highway and seize the key rebel-held town of Jisr al-Shughour in Idlib province, which Assad’s forces lost in April to insurgents that included al-Qaida’s Nusra Front.

The loss of Jisr al-Shughour, followed by the fall of the entire province, was a resounding defeat for Assad, opening the way for rebels to threaten his Alawite heartland in the coastal province of Latakia.