At least 80 Syrian soldiers were killed by rebels over the weekend in clashes and attacks on security forces checkpoints, an activist group says.
Rebels told the Syrian Observatory for Human Rights that more than 100 soldiers had died in the suburbs of Damascus and Idlib province.
The Observatory said it had confirmed the names of 80 dead with local medics.
Meanwhile, the European Council's president has said the EU and Russia must combine their efforts on Syria.
After a summit in St Petersburg with President Vladimir Putin, Herman Van Rompuy admitted they had "divergent assessments" of the situation.
But both sides agreed the peace plan negotiated by the UN and Arab League envoy, Kofi Annan, was the best way to "break the cycle of violence in Syria, avoiding a civil war", Mr Van Rompuy added.
"We need to combine our efforts in order for this to happen, and to find common messages on which we agree. We need to work towards an immediate stop of all forms of violence in Syria."
Mr Van Rompuy's call came as the Syrian Observatory for Human Rights (SOHR) said it had confirmed with local doctors that at least 80 soldiers had been killed in clashes with Free Syrian Army fighters over the weekend.
At least 19 soldiers, eight rebels and 19 civilians were killed in violence on Sunday, the UK-based group added.
Another 89 were killed on Saturday, including 57 soldiers, which the SOHR said was the largest number of fatalities the military had suffered in a single day since the start of the uprising against President Bashar al-Assad in March 2011.
The reports could not be independently confirmed.
SOHR head Sami Abdul Rahman told AFP news agency that there had been a sharp increase in the number of clashes between the army and rebels over the weekend.
"Troops are vulnerable to heavy losses because they are not trained for street battles and are therefore exposed to attacks," he said.
"What exacerbates those losses is that the army is fighting locals of those towns and villages... who know the area inside and out and enjoy public support," he added.
Mr Abdul Rahman said rebels had targeted military vehicles advancing on towns and villages, firing automatic weapons and throwing grenades.
Overnight, there were clashes in the Idlib province, as security forces used tanks, rocket launchers and artillery to bombard the towns of Kafr Nabal, Maarat al-Numan, Ariha and Rama, according to the main opposition coalition, the Syrian National Council.
The SOHR said at least two rebels were killed in Kafr Nabal, while two others died in separate incidents in the Damascus suburbs of Jubar and Douma.
On Sunday, President Assad told parliament that Syria was facing not an internal crisis but an external war, waged against it because of its support for resistance to Israel.
In his first public comment on the massacre at Houla, in which 108 people were killed on 25 May, Mr Assad said that even "monsters" would not have carried out such an act and it should prompt an end to bloodshed.
Survivors and human rights groups blamed the army and shabiha militiamen allied to the government for the deaths.
Mr Assad said the door remained open for dialogue with opposition forces. He excluded those engaged in "terrorist acts" or taking orders from abroad.