A ferocious battle on the outskirts of the northern city of Raqa, near the Turkish border, killed dozens of Syrian troops and rebel fighters on Saturday, a watchdog reported.
"Fierce clashes pitting rebel fighters from several battalions against regular troops have raged since dawn on the outskirts of Raqa city," said the Syrian Observatory for Human Rights.
"Army troops shelled several city neighbourhoods, as well as the outskirts, while the clashes left dozens of troops and rebels dead," the Britain-based group said without giving exact numbers.
"Explosions could be heard in the city, and towers of smoke could be seen rising into the sky."
Both the Observatory and activists in Raqa said the army was using helicopters to strafe rebels in some parts of the provincial capital, in a rare escalation of violence.
And mortar shelling at daybreak forced many people to flee the Masaken Shuhada area of the northern city, which is home to about 800,000 displaced by violence elsewhere in Syria, said the Observatory's Rami Abdel Rahman.
"Because of the large number of civilians who have sought shelter in Raqa from other parts of Syria, the rebel Free Syrian Army had an agreement not to assault the city," he said.
"Saturday's battle was a rare but intense escalation."
Fresh battles also rocked Daraya, a key rebel enclave southwest of Damascus which the army has being fighting to take back from insurgents for weeks, said the watchdog.
The army also pounded the nearby rebel-held town Moadamiyat al-Sham, as well as Douma northeast of Damascus and Yabrud near the Lebanese border.
Elsewhere, clashes broke out between rebels and troops in a sprawling police academy in the northern province of Aleppo, said the Observatory.
It meanwhile cited reports that insurgents stormed the main building at the Khan al-Assal academy, days after the outbreak of a battle for one of the regime's last bastions in the north of Aleppo province.
Saturday's violence came a day after at least 142 people were killed across Syria, among them 46 civilians, said the Observatory, which relies on a network of activists, doctors and lawyers for its reports.
Some 70,000 people have been killed in Syria's nearly two-year war, most of them civilians, the UN says.