Syria's state news agency says gunmen have killed two army colonels in Aleppo, the country's largest city and a stronghold of support for President Bashar Assad.

SANA says four men belonging to an "armed terrorist group" opened fire on the officers at a roundabout in the central city of Aleppo on Thursday. The colonels were on their way to work.

They were identified as Abdel-Karim al-Rai and Fuad Shaban. No one immediately claimed responsibility for the attack.

The anti-Assad uprising has become increasingly militarized since it started last March, but Aleppo has largely been spared clashes between government forces and rebels.

However, two mysterious bombings in the city this month killed 29 people. No group claimed responsibility.