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 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 Rastan at the weekend, despite a UN-backed nominal ceasefire that was supposed to come into effect just over a month ago.
Meanwhile, the EU has imposed another round of sanctions on Syria - the 15th so far - in an effort to increase pressure on the government.
The Observatory said Rastan, which lies 180km (120 miles) north of Damascus, was subjected to sustained shelling overnight, leaving dozens of people injured.
The city, currently an opposition stronghold, has been fiercely contested during the Syrian uprising and control of the town has changed several times.
Separately, the 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, some two-thirds of the total intended for deployment as part of a 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 EU gave no official details of its newly agreed sanctions, but an EU diplomat said the 27-member bloc had agreed to an assets freeze and visa ban on two companies and three people who are believed to be financially backing the government.
British Foreign Secretary William Hague said the ceasefire was "not being fully implemented".
"There continues to be killing, torture, abuse in Syria. So it's very important we keep the pressure on the Assad regime."
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, increasing fears that extremists are taking advantage of the unrest.
The violence also once again ignited tensions in neighbouring Lebanon, where clashes in the northern city of Tripoli over the weekend left three dead, according to local media.