Heavy clashes are raging inside the Syrian rebel bastion of Yabrud near the border with Lebanon a day after government troops entered the town.
Yabrud sits near key rebel supply lines stretching into Lebanon and its fall would deal a major blow to the rebels as the war enters its fourth year.
"Fierce clashes are taking place inside the eastern neighbourhoods of Yabrud," security sources said on Saturday, adding that "the 13 rebel chiefs leading operations are dead".
They said there had been "very many deaths among the insurgents" in the fighting.
Syrian state television broadcast footage from a hill overlooking the town, saying "the Syrian army is progressing in the town and spreading its control over most of the areas linking Syria and Lebanon".
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"The army is intensifying its strikes against the terrorist hideouts," a correspondent for the channel on the ground said, using the government's term to refer to rebels.
He reported "fierce fighting between the army and terrorist groups including al-Nusra Front," the Syrian al-Qaeda affiliate.
The Syrian Observatory for Human Rights said a Kuwaiti commander in the al-Qaeda-linked group known as Abu Azzam al-Kuwaiti was killed in Friday's fighting.
It identified him as the number two al-Nusra commander in the Qalamun region, where Yabrud lies.
He had helped negotiate the prisoner swap that saw rebels release more than a dozen Christian nuns earlier this week.
The Observatory said the fighting was centred on the eastern entrance to Yabrud.
"There is fierce resistance by rebels led by al-Nusra Front, which is trying to defend the town," Observatory director Rami Abdel Rahman said.
The Observatory, which relies on a network of activists and other contacts inside Syria, said regime helicopters were dropping explosives-filled barrels on the outskirts of Yabrud.
Battle-hardened Hezbollah guerrillas from Lebanon were fighting inside the town, it said.
