A suicide car bomb has exploded in the Kurdish city of Qamishli, state television said, killing four in the first such attack in Syria's Kurdish region which has kept out of the conflict between rebels and the regime.
Government forces and rebels, meanwhile, pressed on for the northern city of Aleppo on Sunday - the main battleground since July of Syria's 18-month conflict.
The broadcaster said at least four people were killed in the Qamishli blast, while the Syrian Observatory for Human Rights (SOHR), a London-based activists group, said eight members of the security forces were killed and that the attack targeted their headquarters in the city.
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"A suicide terrorist using a car laden with explosives attacked the western district of Qamishli," the television said.
The SOHR said "at least eight members of the security forces were killed, and 15 were injured," adding that the blast was followed by heavy gunfire.
The Qamishli attack comes less than a week after a twin bomb attack struck the heavily-guarded Syrian army headquarters in the heart of Damascus, killing at least four of its guards. An Islamist rebel group claimed the Damascus attack.
Sunday's bombing was the first time since the outbreak of the anti-regime revolt that Qamishli witnessed such a violent attack, Observatory director Rami Abdel Rahman said.
Abdel Rahman said the military pulled out of Kurdish regions in northeastern Syria, including Qamishli, several months ago and the rebel Free Syrian Army (FSA) had no presence in the city, although some other fighters are based there.
The Qamishli blast came as intense fighting swept Syria's second city Aleppo after a night of heavy shelling that destroyed houses and killed at least three people, including two civilians, said the Observatory.
Aleppo has been gripped by fighting on an unprecedented scale since Thursday and the Observatory, which relies on a network of activists on the ground, said that rebel mortar fire damaged two helicopters at the area's Al-Nairab military airport.
The army, for its part, shelled several other districts of Aleppo and battled rebels in Aleppo's northern district of Jandul, the Observatory said.
"There were many rebels and soldiers killed, but both sides are trying to conceal their casualties," Abdel Rahman said.
In Damascus province, rebels killed nine soldiers when they attacked a military checkpoint on the road linking the capital with Qatana to the southwest, the Observatory reported.
Four people were also killed in the town of Irbin in Damascus province, the Observatory said.
A Kurdish activist, Raad Basho, was gunned down outside his home in the Kurdish city of Hasakeh, said the Observatory, whose nationwide toll for Sunday excluding the Qamishli attack was at least 66 people killed - 34 troops, 22 civilians and 10 rebels.