Syrian rebels in Aleppo (file)
Syrian rebels in Aleppo (file)Reuters

Dozens were killed Wednesday when Syrian rebels set off a powerful tunnel explosion targeting an intelligence headquarters in Aleppo and clashed with regime forces, a monitor said.

The Syrian Observatory for Human Rights said the blast hit a building housing the air force intelligence offices in the west of Syria's second city and was followed by heavy clashes, reports AFP.

"The explosion and the fighting caused dozens of deaths on both sides," the Britain-based group said, adding that part of the building had collapsed.

At least 20 regime forces and 14 rebels were killed in the fighting according to the monitoring group. They added that the attempt to conquer the regime headquarters failed, and sporadic fighting had started to dwindle.

The clashes pitted rebels from several factions against government troops supported by fighters from Lebanese Shi'ite terrorist group Hezbollah, an Iranian proxy and a key backer of Syrian President Bashar al-Assad, it said.

An AFP journalist in eastern Aleppo said the blast was loud enough to be heard across the city and some residents said it had felt like an earthquake.

A Syrian military source confirmed the explosion and said clashes were continuing.

"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 source said.

"There are now ongoing clashes and the Syrian air force is hitting the positions of the gunmen in the area," the source said.

Al-Qaeda's affiliate in Syria, the Nusra Front, said on Twitter that its forces, along with other rebel factions, had "stormed the air force intelligence offices and surrounding buildings."

Setting off explosives from tunnels dug into government-controlled areas has become a favored tactic of Syria's rebels, especially in Aleppo.

A similar blast from explosives planted in a tunnel under Aleppo's Old City last December killed at least seven government troops.

Ceasefire efforts stalled

Rebels also last May detonated explosives under the city's famed Carlton Hotel, which government forces had been using as a base.

Fighting in Aleppo erupted in mid-2012, and control of the city - once Syria's main commercial hub - has since been divided between rebels on the eastern side and the regime in the west.

Wednesday's attack came after rebel forces in Aleppo rejected a UN plan to freeze fighting in the northern city.

UN envoy Staffan de Mistura has made the plan for a temporary ceasefire in Aleppo the centerpiece of his efforts to bring any kind of halt to the fighting in Syria, where more than 220,000 people have been killed since conflict erupted in March 2011.

De Mistura in October unveiled the proposal to suspend fighting in Aleppo to allow humanitarian aid deliveries and make a first step toward a broader political deal.

But rebel representatives on Sunday refused to consider the proposal unless it forms the basis for a "comprehensive solution" to the conflict through the departure of Assad.

De Mistura has angered the opposition by describing Assad as "part of the solution" to the Syrian conflict.

Some Western powers have also cast doubt on the plan, with the French ambassador to the United Nations, Francois Delattre, saying Tuesday that "France remains skeptical about the regime's willingness" to follow through on it.