Activists say there are intense clashes in the northern Syrian city of Aleppo after militants attacked a government-held neighborhood, capturing several buildings.

The Local Coordination Committees and the Aleppo Media Center say Tuesday's fighting is concentrated in the western Zahra neighborhood.

Opposition fighters and Islamic militants have launched a series of attacks in Aleppo since last week capturing the city's Scientific Research Center that was used as a military base.

The LCC and the Britain-based Syrian Observatory for Human Rights say the attack on Zahra started Monday night with a large explosion that killed government troops and pro-government gunmen.

The Observatory says it was a suicide attack carried out by a member of al-Qaida's branch in Syria, the Nusra Front. It says the blast killed 25 troops and pro-government gunmen.