Fifteen soldiers, including two colonels, died in an ambush by rebels at dawn on Wednesday in northern Aleppo province, the Britain-based watchdog said.
The ambush took place near the village of Al-Rai, scene of clashes between opposition fighters and regime forces, the monitor said, adding that two rebel fighters were also killed.
"Syrian regime forces have scaled up military operations in Aleppo in recent days," after a lull in violence following a truce that came into force on April 12, the Observatory said.
Meanwhile, in the Harasta suburb of Damascus, "violent clashes" killed at least six soldiers, the monitoring group added.
In Daraya town in Damascus province, two civilians were killed by soldiers, the organisation said. Another civilian in the same town died Wednesday from wounds he sustained several months ago, the NGO reported.
The eastern province of Deir Ezzor also suffered violence, with shelling by government forces on Al-Dahla village, the Observatory reported. In Al-Quriya town, troops conducted raids and burned down activists' homes, it added.
And in southern Daraa, a civilian was killed by loyalist gunfire in the Al-Lajaa area, the Observatory reported.
Troops were also carrying out raids and arrests in Harasta as well as Zabadani, near Damascus, it said.
The violence took place amid a putative ceasefire that came into effect on April 12, with the United Nations accusing both sides of violating it.
A peace plan brokered by UN-Arab League envoy Kofi Annan calls for a daily two-hour humanitarian ceasefire, media access to all areas affected by the fighting, an inclusive Syrian-led political process, a right to demonstrate and the release of detainees.
According to the UN, more than 9,000 people have been killed in Syria since an anti-regime uprising broke out in March last year, while the Observatory puts the figure at more than 11,100.
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"Testing the Olympic Park and its operations is a hugely important part of our plans," said London Games chairman Sebastian Coe.
"Over the last 10 months, over 250,000 spectators have watched world-class sport as part of the London Prepares series test event programme, and, in doing so, every one of them is helping us deliver the best possible Games."