Syrian soldiers and rebel forces are clashing at the eastern outskirts of the town of Yabroud, the last rebel bastion near the Lebanese border north of Damascus.

Beirut-based station al-Mayadeen broadcast footage showing soldiers charging through a field towards an arched entrance of the town on Saturday and a sign saying "Welcome to Yabroud".

Gunfire could be heard as the soldiers advanced.

The Syrian Observatory for Human Rights, a Britain-based watchdog group, said army forces were advancing with support from Lebanon's Shia movement Hezbollah, a staunch regime ally.

A military source said "fierce clashes" were taking place between army and rebel forces inside Yabroud.

"The 13 rebel chiefs leading operations are dead," he added, saying there were "very many deaths among the insurgents."

Syria's state television broadcast images of the town from its outskirts, reporting that "the Syrian army is progressing in the town".

Capturing Yabroud, located on a key rebel supply route into nearby Lebanon, would be a major victory for the regime of President Bashar al-Assad.

Earlier today, Rami Abdel Rahman, head of the Syrian Observatory, told the AFP news agency that regime helicopters were dropping explosives-filled barrels on the outskirts of Yabroud, while Hezbollah fighters battled inside the town.

"Heavy fighting is taking place at the eastern entrance of Yabroud between the rebels on one side and the Lebanese Hezbollah and army on the other, accompanied by intense bombardment by regime helicopters," Rahman said.

"There is fierce resistance by rebels led by Jabhat al-Nusra [the Syrian al-Qaeda affiliate], which is trying to defend the town."

After a month of aerial bombing and combat around the town, Hezbollah and Syrian troops have captured the areas surrounding Yabroud.

'Nusra leader killed'

The Syria Observatory also said on Saturday that Abu-Azzam al-Kuwaiti, the Kuwaiti commander of the al-Qaeda-linked Jabhat al-Nusra, was killed late on Friday in fighting around the town.

Amer al-Qalamouni, an activist in the area, and the Syrian Observatory said that al-Kuwaiti was a key mediator for the release of a dozen nuns held by rebels earlier this week.

Syrian state news agency SANA had said that government forces captured the eastern and northern entrances of Yabroud, a claim denied by activists.

The town is near the highway linking Damascus to the former commercial hub Aleppo in the north and to the Mediterranean coast in the west, a stronghold of Assad's minority Alawite sect.

Thousands of people have fled Yabroud, a town of an estimated 40,000-50,000 people roughly 60km north of Damascus, and the surrounding areas after it was bombed and shelled last month ahead of the assault. Most of them are now sheltering in and around the Lebanese border town of Arsal.

The government has been making incremental gains along the highway as well as around Damascus and Aleppo in recent months, regaining the initiative in a conflict which enters its fourth year this month.

More than 140,000 people have been killed and 2.5 million have fled abroad as refugees in an increasingly sectarian civil war which began with mass street protests against Assad in March 2011 but turned into an armed conflict after a crackdown on demonstrators.