A truce between Syrian regime forces and rebel groups in three key towns ended early Saturday as the warring parties resumed clashes and shelling, a monitor and a mediator said.
Pro-regime forces, including Lebanon's Shiite Hezbollah militia, had agreed on a 48-hour ceasefire until dawn on Saturday in the rebel bastion of Zabadani and the government-held villages of Fuaa and Kafraya.
"The ceasefire has collapsed in Zabadani, Fuaa, and Kafraya this morning," said Rami Abdel Rahman, head of the Britain-based Syrian Observatory for Human Rights monitoring group.
A Kafraya resident speaking to AFP by phone said "dozens of shells have fallen on the town since early morning".
Abdel Rahman said "there are clashes and shelling in Zabadani, and opposition fighters are shelling Fuaa and Kafraya".
He had no details on who began firing and whether there were any casualties.
Fuaa and Kafraya, the last two regime-held villages in Syria's northwest Idlib province, have been surrounded by a rebel alliance including Al-Qaeda's affiliate Al-Nusra Front.
The siege came in retaliation for a fierce offensive on Zabadani, along Syria's border with Lebanon, by pro-regime forces early last month.
Warring parties were negotiating to reach a broader deal including rebel fighters leaving Zabadani and the evacuation of civilians from Fuaa and Kafraya, whose residents are minority Shiite Muslims.
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But the talks failed overnight.
The resumption of hostilities was confirmed by Mohammad Abu Qassem, secretary general of Syria's Tadamun (Solidarity) Party who was mediating the truce.
"The truce has ended, the negotiations have failed, and military operations have resumed in Zabadani, Fuaa, and Kafraya," he told AFP.
This marks the second time a ceasefire for the three towns collapse this month.
The rebels had also sought the release of prisoners held by the regime.
Syria's multi-front war has seen ongoing fighting among regime forces, Kurdish militia, rebel groups, and jihadists including the extremist Islamic State group.
IS jihadists have clashed with government forces, Kurdish fighters, and rebel groups.
Elsewhere, the Observatory reported that a fierce attack by IS on the rebel stronghold of Marea in northen Aleppo province had killed dozens.
Abdel Rahman said at least 32 rebel fighters and 20 IS jihadists -- three of them suicide bombers -- died in the assault including a suicide bombing that killed six civilians.
More than 240,000 people have been killed since Syria's conflict began in March 2011, and half of the country's population has been displaced by the war.