A day-old truce in a besieged rebel-held town near Damascus broke down Thursday as Syrian warplanes bombed the divided northern city of Aleppo for a 12th straight day, activists said.

By Wednesday, the Aleppo air blitz that began on December 15 had killed at least 422 people, mostly civilians, according to the Syrian Observatory for Human Rights, a Britain-based group relying on activists and other sources inside the war-torn country.

In Moadamiyet al-Sham, near the capital, clashes broke out on Thursday afternoon, a day after opposition and regime sources announced a truce for the town, which had been under a suffocating army siege for a year.

The opposition blamed President Bashar al-Assad's troops for breaking the truce.

"They opened heavy machine-gun fire without any reason. It means there are people from the regime who don't want the siege on our town to be lifted. They are trying to end the truce in any way possible," Ahmad, a local activist, told AFP via the Internet.

The Syrian Revolution General Commission, a network of activists on the ground, confirmed the fighting, and said the army was sending "heavy reinforcements" towards the town.

On Wednesday rebels raised the national flag above the town in accordance with a ceasefire deal that was supposed to allow food in, but Ahmad said none had arrived.

Negotiations were under way for another truce in Barzeh, northern Damascus, according to activist Emad al-Barzawi, but he said "there has been no decision yet."

Barzeh has come under frequent bombardment in recent months, forcing hundreds of residents to flee.

In Aleppo, the country's second city and onetime commercial hub, the airforce kept up its offensive a day after 12 people were killed in aerial attacks in and around the city, said the Observatory.

Regime aircraft launched a fresh attack using TNT-packed barrels against the city's Hanano district, and another air strike against Daret Ezza in the surrounding province, according to the Observatory.

The European Union, the United States and the Arab League have condemned the air force's use of barrel bombs, and the US-based Human Rights Watch has described their use as "unlawful" because they do not discriminate between civilians and fighters.

"When the bombing starts, you feel like you're going to die any second," Abu Omar, an activist in the town of Marea near Aleppo, told AFP via the Internet.

"The regime sees us all as terrorists -- fighters, civilians, men, women, children. To them, anyone who lives in a liberated (rebel) area is a terrorist."

An estimated 126,000 people have been killed since the start of Syria's uprising, which began with peaceful protests in March 2011 but escalated into a civil war after regime forces fired on demonstrators.

The regime has always referred to the opposition as "terrorists," even before the rise of powerful jihadist groups among the rebels.

Pro-regime newspaper Al-Watan said the army had carried out "door-to-door operations" on Wednesday in Adra, northeast of Damascus, where it said 57 "terrorists" have been killed.

State news agency SANA meanwhile said Islamist rebels assassinated a Muslim cleric in Damascus province "while he was on his way out of the mosque after evening prayers."