Heavy clashes were underway Saturday at the entrance to the Syrian rebel bastion of Yabrud, a monitoring group said, a day after a military source said troops had entered the town.Yabrud sits near key rebel supply lines stretching into nearby Lebanon, and its fall would deal a major blow to the rebels as the war enters its fourth year, with 146,000 people dead nationwide and millions more displaced."Heavy fighting is taking place at the eastern entrance of Yabrud between the rebels on one side and the Lebanese Hezbollah and army on the other, accompanied by intense bombardment by regime helicopters," Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, told AFP."There is fierce resistance by rebels led by Al-Nusra Front (the Syrian Al-Qaeda affiliate), which is trying to defend the town."The Britain-based 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, while Hezbollah fighters experienced in guerrilla warfare battled inside the town.After a month of aerial bombing and combat around the town, Hezbollah and Syrian troops have captured all the hills overlooking it. On Friday a military source told AFP troops had entered Yabrud and "advanced along the town's main street" as rebels fled towards a village to the south.Al-Nusra Front admitted on Friday that "one position" had fallen, "causing brother fighters to fall back to rear bases," but denied rebels were retreating and said reinforcements were on the way.The Observatory 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, and said he had helped negotiate the prisoner swap that saw rebels release more than a dozen Christian nuns earlier this week.The nuns from the historic Christian town of Maalula -- where residents still speak the Aramaic language of Jesus Christ -- were freed unharmed in exchange for prisoners held by the regime after being abducted in December.The battle for Yabrud is vital for Hezbollah, which first acknowledged last spring that its forces were fighting alongside those of Assad.Hezbollah wants to sever a key rebel supply line to the Sunni town of Arsal across the border in eastern Lebanon.It says car bombs that have been used to attack it inside Lebanon were loaded with explosives in Yabrud and then driven via Arsal to their targets.