Activists and Syrian state television reports that soldiers and civilians were killed in two explosions.
Twenty-five people were killed and 175 people were wounded in two blasts targeting security bases in Syria's northern city of Aleppo on Friday, state television quoted the Health Ministry as saying.
The opposition Syrian Observatory for Human Rights quoted residents as saying three blasts occurred, including one near a security forces' building in the city. They also reported hearing gunfire.
A resident contacted by Reuters said an area around a military intelligence complex in the New Aleppo district had been sealed off after the explosions.
Aleppo, Syria's main commercial hub, had been relatively quiet during the 11-month uprising against President Bashar Assad but has seen increasing protests and violence in recent weeks.
On Dec. 23, suicide car bombers struck Damascus
in what was then the bloodiest violence in the capital since the revolt against Assad began.
At least 44 people were killed then and the government blamed al Qaida for the attack, which took place one day after the arrival of an Arab League observer mission.
On Jan. 6, a suicide bomber killed 26 people and wounded 63 in Damascus. Syria's Interior Minister Ibrahim al-Shaar vowed an "iron-fist response" to the attack.