BEIRUT: A large explosion went off near an office of the ruling Baath party in Syria's second city of Aleppo on Friday evening, said the Syrian Observatory for Human Rights.
"A big explosion was heard near Al-Jabiri Square. It targeted an office of the Baath party, according to initial reports," the Britain-based watchdog said in a statement.
Contacted by AFP on the phone from Beirut, the Observatory's Rami Abdel Rahman said that, so far, there no reports of any casualties from the blast.
But he added that unidentified gunmen shot dead a guard outside the Baath party office as they drove by in a vehicle soon after the explosion.
"The blast was powerful but we do not know what was the origin of the explosion," said Abdel Rahman. "We can confirm one death -- the guard of the Baath party office."
An activist on the ground in the northern city, Mohammed al-Halabi, told AFP the blast happened near a park which regime forces had emptied out of before it went off at around 8:45 pm (1845 GMT).
He said there was a chance that the explosion was caused by a sound bomb.
The blast came only hours after forces loyal to President Bashar al-Assad opened fire on protesters during a demonstration Aleppo, killing one civilian, the watchdog said.
Also on Friday, Syrian state television said the army foiled a would-be suicide attack in Aleppo by a bomber whose car was laden with 1,200 kilos (2,640 pounds of explosives.
Anti-regime activists dismissed the official account of a foiled attack as "a lie," the activist Halabi told AFP via Skype.
Traditionally a bastion of the Baath party, anti-regime sentiment has been on the rise in the commercial hub of Aleppo in recent months.
Meanwhile an explosion was heard near an army checkpoint in northwestern Idlib, the Observatory said.
The attacks came one day after twin bombings in Damascus left at least 55 dead and almost 400 injured.
An anti-regime uprising in Syria began as a peaceful popular revolt but has turned into an insurgency amid mounting calls to arm rebels seeking to overthrow Assad, who heads the Baath party.
More than 12,000 people, the majority of them civilians, have died since the uprising began, according to the Observatory, including more than 900 killed since a UN-backed truce went into effect on April 12.