Unidentified gunmen killed six Syrian soldiers in the northeast, as regime forces shelled areas in Homs and Aleppo on Tuesday, monitors said, on the day a UN-Arab peace plan is due to start taking effect.
The attack on regime forces occurred between the villages of Masaada and Marqada, in the province of Hassakeh, according to the Syrian Observatory for Human Rights, a Britain-based monitoring group.
Syrian forces shelled several neighbourhoods of the flashpoint central city of Homs, including Khaldiyeh, as well as the village of Marea, and neighbouring Hawr al-Nahr village, in the northern province of Aleppo, the centre said.
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Regular forces also carried out arrests in the village of Kfar Zeita, in the central province of Hama, a day after fierce clashes between regime troops and rebel fighters, the Observatory reported.
Meanwhile, explosions were heard outside Douma, a northern suburb of the capital, the centre said.
The Local Coordination Committees, one of the main opposition groups inside Syria, said "large military reinforcements" had arrived overnight on the eastern outskirts of Rastan, in the central province of Homs.
The reports, which cannot be verified due to curbs on foreign media, came after one of the bloodiest days in Damascus' crackdown on Arab Spring-inspired protests that has seen some people take up arms against the regime.
Under the peace plan agreed with UN-Arab League envoy Kofi Annan, the Syrian government is supposed to withdraw its forces from population centres on Tuesday ahead of a ceasefire deadline on Thursday.