At least 30 people - including 23 soldiers - have died in heavy overnight clashes in the central Syrian city of Rastan, according to activists.
The Syrian Observatory for Human Rights said dozens of others were wounded in the city, in the restive Homs province.
Three troop carriers were destroyed in fighting, the UK-based group said.
If confirmed, the assault 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.
The city was subjected to sustained shelling overnight, the Observatory said.
Rastan, currently an opposition stronghold, has been fiercely contested during the Syrian uprising and control of the town has changed several times.
Meanwhile, the Syrian army has raided a Sunni village north of Hama, killing five people, the Observatory said.
Activists 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.
The UN on Sunday said it had 189 observers in Syria, two thirds of the total intended for deployment as part of the six-point peace plan mediated by UN-Arab League envoy Kofi Annan.
The BBC's Jonathan Head in neighbouring Turkey says neither the Syrian military nor the opposition appears to have any confidence that the plan will hold, with both using the putative ceasefire to gain ground before full-scale fighting resumes.
The UN estimates at least 9,000 people have died since pro-democracy protests began in March 2011.
On Saturday, a radical Islamist group said it carried out a massive bomb attack in Damascus last week, stoking fears that extremists are taking advantage of the unrest in Syria.