A ferocious battle on the outskirts of the northern city of Raqa, near the Turkish border, killed at least 16 rebels and 10 soldiers 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," the Britain-based group said.
Observatory director Rami Abdel Rahman said it was impossible to provide a final death toll, though he could confirm that at least 26 fighters on both sides had been killed.
Both the Observatory and activists in Raqa said the army was using helicopters to strafe rebels in some parts of the provincial capital.
The battle began after rebels assaulted several army checkpoints in the city, state news agency SANA said.
"Troops repelled an attempted attack by armed terrorists who attacked and staged an attempted assault in the northeast of Raqa city," it said.
"The terrorists tried to attack army checkpoints near the central prison."
The Observatory also said that mortar fire at daybreak forced many people to flee the Masaken Shuhada area of Raqa, which is home to about 800,000 people displaced by violence elsewhere in the country.
"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," Abdel Rahman 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 Observatory.
The army also pounded the nearby rebel-held town of Moadamiyet 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 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.
Some 70,000 people have been killed in Syria's nearly two-year war, most of them civilians, the United Nations says.