Twelve members of the Syrian security forces and a civilian were killed in clashes on Monday in several regions, monitors said, a day before a truce is supposed to start taking effect.
Rebel fighters besieged a checkpoint at the village of Salamah, in Aleppo province on the border with Turkey, killing six members of the security and customs services, said the Syrian Observatory for Human Rights.
At least eight rebels who were wounded in the fighting fled across the frontier, said the Britain-based monitoring group.
Clashes were also taking place elsewhere in Aleppo province, with forces loyal to President Bashar al-Assad shelling and storming the town of Tal Rifaat, and unleashing heavy gunfire on the village of Marea.
Two policemen were killed in clashes the neighbourhood of Sukari, in the northern city of Aleppo, the Observatory said.
But SANA state news agency gave a different version of the incident and a much higher toll.
It said 10 policemen and a civilians were killed and 11 others wounded, along with two civilians, when security forces came under fire at a demonstration in Sukari, in the northern city of Aleppo.
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Police had gone to the area to provide protection for the rally, it added.
The Observatory also reported that four four soldiers were killed when an explosion struck a bus carrying troops near the village of Kawkab outside Damascus, while security forces launched a campaign of arrests in the capital's neighbourhood of Kfar Sousa.
In the eastern province of Deir Ezzor, regime forces stormed the village of Muhsen, while clashes took place in the neighbouring village of Al-Bou Amr, and gunfire was heard in the city of Deir Ezzor itself, the group said.
In Daraa, a civilian was shot dead in an ambush near the village of Saida, the Observatory said.
Almost 180 people, mostly civilians, were killed in weekend violence, ahead of the UN deadline of Tuesday for regime forces to cease fire, as agreed with special envoy Kofi Annan.
Under a peace deal brokered by the former UN chief, the Syrian army was scheduled to withdraw from protest cities on Tuesday, with a complete end to fighting set for 48 hours later.
But the truce appears in jeopardy after Damascus said it would only carry its side of the bargain if rebels first handed over written guarantees to stop fighting, a demanded rejected by rebel army chief Colonel Riyadh al-Asaad.