A 72-hour ceasefire came into effect today in the last rebel stronghold along Syria's border with Lebanon, according to a resident and a monitoring group.

The Syrian Observatory for Human Rights said rebel groups and pro-regime factions, including Lebanon's Shia militia Hezbollah, had agreed on a 48-hour ceasefire late last night.

It was extended this afternoon for another 24 hours, observatory head Rami Abdel Rahman said.

"No shots have been fired since 6:00 am" local time (4am Irish time) in Zabadani as well as in Fuaa and Kafraya, two regime-controlled villages in northwestern Syria, Abdel Rahman said.

"We really noticed that it was relatively calm this morning," Mohammad, a Zabadani resident, told AFP.

"We didn't hear sounds of shelling or clashes, and we hope the situation stays like this."

Pro-regime forces launched an offensive to seize Zabadani from rebel groups early last month.

In retaliation, a rebel alliance including Al-Qaeda affiliate Al-Nusra Front surrounded Fuaa and Kafraya, two Shia Muslim villages in Idlib province, and regularly fired rockets into them.

Mohammed Abu Qassem, secretary general of Syria's Tadamun (Solidarity) Party, told AFP he had negotiated the ceasefire on behalf of fighting groups inside Zabadani.

"Tadamun was authorised to negotiate with the government to reach a new agreement," Abu Qassem said.

"Since the beginning of the military operation, we have been trying to find a solution to the crisis in Zabadani," he said, adding that a local administrative council, rebel groups and regime forces had signed off on the ceasefire.

He said intensifying rebel attacks on Fuaa and Kafraya had expedited the agreement.

"We accepted the ceasefire because we wanted to end the battle with as few losses as possible," a security source told AFP.

"But we won't accept that the armed groups stay in Zabadani after today," he said.

According to the observatory, negotiations are ongoing regarding the safe exit of rebel fighters from Zabadani, as well as the provision of food and medical aid to residents of Fuaa and Kafraya.

Local ceasefires have been implemented intermittently in parts of Syria, often to bring in humanitarian aid to besieged populations.

At least 240,000 people have been killed since Syria's conflict erupted in March 2011.