Syrian government warplanes have bombed a northern town near the Turkish border, killing 21 people, as heavy fighting spread within Aleppo's historic old city.

The Britain-based Syrian Observatory for Human Rights said the death toll from Monday's attack in Salqin included eight children. In a video released by activists from the town, a number of the victims are seen piled in the back of a pick-up truck.

Another activist group, the Local Coordination Committees, said the attack killed 30 people. There was no independent confirmation of the casualties from the bombing because Syria restricts reporting by international journalists.

Deadly violence was reported across the country, including government shelling in Hama, Daraa and Homs provinces.

Also Monday, fires that gutted large parts of Aleppo's medieval Souk al-Medina, a vast covered market, broke out in other areas of the Old City as rebels and government forces engaged in fierce fighting.

Traders and visitors to the city described the fires, which began on Saturday, as a side effect of the clashes in the souk. Aleppo, one of Syria's greatest historic and commercial assets, was once the last stop before Europe for traders on the ancient Silk Route from Asia.

Meanwhile, the Damascus representative of the new international envoy for Syria said the large number of deeply divided rebel groups is one of the main obstacles to the U.N. mission's efforts to end the country's 18-month crisis.

Mokhtar Lamani, who represents special envoy Lakhdar Brahimi in the Syrian capital, also pointed to “huge fragmentation” within Syrian society, which he said contributes to a high level of mistrust. He said “many elements that have nothing to do with Syria” are taking advantage of the situation.

On Sunday, Syria's civil war appeared to widen to the country's mainly Kurdish northeast as a suicide car bomber killed several people in the town of Qamishli, near the Turkish border.

It was the first such attack in the predominantly-Kurdish area since the outbreak of the Syrian uprising against President Bashar al-Assad's autocratic rule. Syrian Kurds largely have stayed out of the conflict.