At least 23 soldiers have died in clashes in the central Syrian city of Rastan, according to activists.
The UK-based Syrian Observatory for Human Rights said dozens were wounded in the attack in the city, in the restive province of Homs.
Three troop carriers were destroyed in fighting, the group said.
If confirmed, the attack would be one of the deadliest suffered by security forces in the 14-month-long uprising against President Bashar al-Assad.
It comes after government forces launched a fresh assault on the city at the weekend, despite a UN-backed nominal ceasefire that was supposed to come into effect just over a month ago.
Dozens of people have been wounded in shelling in the city, the Observatory said.
At least 30 people died on Sunday - mainly civilians - as violence surged at flashpoints across the country despite an increase of UN observers.
The figures cannot be verified independently, as journalists' movements are severely restricted in Syria.
On Sunday, the UN said it had 189 observers on the ground.
The UN estimates at least 9,000 people have died since pro-democracy protests began in March 2011.