Pro-government fighters in the village of al-Sahel, near Yabrud.

Pro-government fighters in the village of al-Sahel, near Yabrud. Photo: AFP

Damascus: Syrian troops have advanced in the key rebel bastion of Yabrud as the country's civil war enters its fourth year, with more than 146,000 dead, millions displaced and peace efforts stalled.

The Syrian Observatory for Human Rights NGO said army forces were advancing with support from Lebanon's Shiite movement Hezbollah, a staunch regime ally.

A military source said "13 rebel chiefs leading operations are dead", adding there were "many deaths among the insurgents".

A view of part of the town of Yabrud, a rebel stronghold near the Lebanese border.

A view of part of the town of Yabrud, a rebel stronghold near the Lebanese border. Photo: Reuters

State television broadcast images of the town from its outskirts and said troops had advanced two kilometres inside the town.

Yabrud is a key rebel supply route and their last stronghold in the Qalamun region along the border with Lebanon and on the highway between Damascus and third city Homs.

The latest battle illustrates the intractability of the conflict that began on March 15, 2011, after popular uprisings that toppled dictators in Tunisia and Egypt.

Two years later, the conflict appears to have reached stalemate, with some predicting it could last another 10 or 15 years.

Rebels control large swathes of Syria, but are fighting both the regime and an al-Qaeda-inspired group they once welcomed.

Loyalist forces hold the more densely populated regions, and are advancing south of Damascus, in the Qalamun region and in Aleppo in the north, seeking to protect the coast, major towns and key roads.

Syria's mostly exiled political opposition has secured Western recognition but is largely ignored by rebels on the ground.

On Saturday, Syrian National Coalition chief Ahmad Jarba urged international backers of the opposition to make good on promises to supply the rebels with better weapons.

"We renew our request to the friends of the Syrian people to immediately keep their promises ... to do with qualitative weapons," he said.

"Our fighters are not only facing regime forces and their allied gangs," he said in a speech in Istanbul. "They are also facing extremist gangs and are cleaning our house of terrorist mercenaries, particularly the Islamic State of Iraq and the Levant (ISIL)."

On the ground, both sides have been accused of abuses.

The human cost of the conflict has soared, with 9 million people forced from their homes, creating the world's largest displaced population, according to the UN's refugee agency.

More than 2.5 million Syrians are registered or awaiting registration as refugees in neighbouring countries, and in excess of 6.5 million people are displaced inside the country.

AFP