"A suicide terrorist using a car laden with explosives attacked the western district of Qamishli," the television said.
The Britain-based Observatory 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.
"The earth shook beneath us, the force of the explosion was immense," an activist who identified himself as Serdar told AFP via the Internet.
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.
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, and a Kurdish activist, Raad Basho, was gunned down outside his home in the Kurdish city of Hasakeh in the northeast.
The northern province of Deir Ezzor, Hama in central Syria and Daraa in the south came under heavy shelling by regime forces, the Observatory said.
It reported a total of at least 114 people killed in violence across the country on Sunday, including 57 who died in Damascus province and 39 in Deir Ezzor.
-- 'History will not forgive'
In Aleppo on Sunday, a huge plume of black smoke billowed over the ancient market quarter as fire devoured the wares and wooden fittings of the historic souk of Syria's commercial capital.
Shopkeepers could only look on as the inferno sparked by weekend clashes between troops and rebels ripped through textile and perfumery sections of the market, ravaging family businesses dating back generations.
"I inherited my store from my father," one shopkeeper told an AFP journalist in the city. "This is a dirty war, and we are the ones losing out."
Before the rebellion against President Bashar al-Assad's regime rule erupted in March last year, the UNESCO-listed covered market would have been packed with tourists.
The market quarter is one of the centrepieces of Aleppo's Old City, listed by the UN cultural organisation as a world heritage site since 1986.
On the political front, Turkish Prime Minister Recep Tayyip Erdogan, a one-time ally of President Bashar al-Assad turned critic, reiterated on Sunday that China, Iran and Russia must stop aiding the Damascus regime.
"Please rethink your current attitude. History will not forgive those who have sided with these brutal regimes," Erdogan said in a speech at a congress of his ruling Justice and Development Party.
Russia, a traditional Damascus ally, and China have blocked all UN Security Council resolutions on the conflict that according to activists has now left around 30,000 people dead in 18 months. The UN puts the toll at 20,000.
Shiite-led Iran is Syria's closest regional ally and is accused by several Western and Sunni-led Arab nations of providing military aid to Assad's regime.
Iraq's Foreign Minister Hoshyar Zebari on Sunday vowed his country would stop and search any flights from Iran over its territory suspected of carrying weapons to Syria, as requested by the United States.