Syrian rebels launched a fierce attack Wednesday on an intelligence headquarters in Aleppo, leaving at least 34 dead, days after the opposition rejected a UN ceasefire plan for the city.

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 Britain-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 explosion, 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 militant group Hezbollah, a key backer of Syrian President Bashar al-Assad, it said.

The assault was repelled with the help of regime air strikes on rebel positions and the clashes eventually subsided.

"Dozens of (rebel) gunmen were killed in artillery and air strikes. The situation is quiet now in the area. There are minor sporadic clashes," the Syrian military source said.

Al-Qaeda's affiliate in Syria, Al-Nusra 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.

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

He held talks in Damascus on Saturday to try to finalise a deal, with a delegation member saying he hoped to set in motion as soon as possible a plan to halt fighting in Aleppo for six weeks.

But rebel representatives 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 sceptical about the regime's willingness" to follow through on it.

Wednesday's attack featured a favoured tactic of Syria's rebels -- especially in Aleppo -- of digging tunnels near government buildings and setting off explosives.

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

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 commercial hub -- has since been divided between rebels on the eastern side and the regime in the west.

The air force intelligence headquarters in Aleppo is a key strategic site for regime forces and the surrounding area has come under repeated attack from rebel forces.