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Hussein Malla, Associated Press
A Lebanese Sunni cleric, center, smiles as he gestures to a Russian security man, not seen, on the top of the Russian embassy building, to stop taking pictures, during a sit-in against Russian intervention in Syria, in front the Russian embassy, in Beirut, Lebanon, Wednesday, Oct. 14, 2015. Moscow, which began its airstrikes in Syria on Sept. 30, said it is targeting Islamic State and other militant groups. But it has also struck positions of rebel groups supported by the West and other regional players.

BEIRUT — Iran has sent hundreds of troops into northern and central Syria, its first such deployment in the country's civil war, joining fighters from its Lebanese ally Hezbollah in an offensive against rebels and taking advantage of cover from Russia's air campaign, a regional official and Syrian activists said Wednesday

Their arrival is almost certain to fuel a civil war in Syria which has already claimed the lives of more than 250,000 people and displaced half of the country's population. It also highlights the far-reaching goals of Russia's military involvement in Syria.

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

The arrival of Iranian troops reinforces fighting under the cover of Russian air power further strengthens the view that Russia's main goal is to shore up Syrian President Bashar Assad's embattled government.

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

He said the Iranian fighters were arriving at Damascus airport and transported to a military base in the coastal town of Latakia from where they were deploying on the ground — mainly in the central Hama province and northern Aleppo province.

He spoke on condition of anonymity because he was not authorized to discuss military affairs.

The Syrian Observatory for Human Rights also reported Wednesday that Iranian troops were arriving in Syria and were transported to the military base in the town of Jableh.

Iranian and Syrian officials have long maintained that Iran has advisers and military experts in Syria. Wednesday's news is the first confirmation of Iranian fighters taking part in combat operations in Syria.