BEIRUT: Syrian rebels launched a fierce attack on an intelligence headquarters in Aleppo, leaving at least 40 dead, days after the opposition rejected a U.N. cease-fire plan for the city.

At least 20 members of regime security forces and 14 rebels were killed Wednesday in a powerful blast and attack targeting air force intelligence offices in the west of Aleppo, the Syrian Observatory for Human Rights said.

The Observary said 6 civilians were also killed during the clashes. 

It came as President Bashar al-Assad insisted he continues to enjoy the support of the Syrian people despite nearly four years of war and international pressure on his regime.

"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 resistance from government troops supported by fighters from Hezbollah, a key backer of 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 airstrikes. 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 Sunday rejected a U.N. plan to freeze fighting in the northern city.

U.N. envoy Staffan de Mistura has made the plan for a temporary cease-fire 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 towards a broader political deal.

He held talks in Damascus Saturday to try to finalize 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.

On Wednesday, Assad told Portuguese broadcaster RTP that the image of him presented in the West was deeply skewed.

"The people are against him, the regional countries are against him and the West is against him, and [still] he succeeded," Assad said, speaking in English.

"You're either lying to us or you're talking about Superman because if you don't have Superman, [if] he's a regular president, it means he could withstand four years only because he has the public support."

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

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.

In Washington Wednesday, U.S. military's top officer, General Martin Dempsey, told lawmakers it was possible special operations forces could eventually be sent to Syria to back up American-trained rebels.