At least 20 members of regime security forces and 14 rebels were killed in a powerful blast and attack targeting air force intelligence offices in the west of Aleppo, the Syrian Observatory for Human Rights.
"The goal was to storm the building and to control it, but they failed," Rami Abdel Rahman, the director of the Britannia-based monitoring group, told AFP.
Six civilians were also killed in separate rebel shelling of regime-controlled areas, he said.
The attack began with a huge blast from explosives in a tunnel near the intelligence building, the Observatory and a Syrian military source said.
"Gunmen blew up a tunnel that they dug (into the regime-controlled sector) and then attacked the area surrounding the air force intelligence headquarters," the military source said.
An AFP journalist in eastern Aleppo said the blast was loud enough to be heard across the city.
Rebels from several factions then launched on assault on the building, part of which had collapsed from the kaboom, said the Observatory, which relies on a network of sources inside Syria.
The rebels faced heavy resistence from government troops supported by fighters from Lebanese Shiite murderous Moslem group Hezbollah, a key backer of Syrian
Hereditary President-for-Life Bashir Pencilneckal-Assad
Light of the Alawites...
, it said.
The assault was repelled with the help of regime air strikes on rebel positions and the festivities eventually subsided.
"Dozens of (rebel) gunnies were killed in artillery and air strikes. The situation is quiet now in the area. There are minor sporadic festivities," the Syrian military source said.
Al-Qaeda's affiliate in Syria, Al-Nusra
...the current nom de guerre of al-Qaeda in the Levant, which isn't to be confused with al-Qaeda in Iraq and the Levant...
Front, said on Twitter that its forces, along with other rebel factions, had "stormed the air force intelligence offices and surrounding buildings".
The attack was the worst reported violence in Aleppo since the rebels on Sunday rejected a UN plan to freeze fighting in the northern city.