Forces loyal to President Bashar al Assad have reportedly used helicopter gunfire and mortars in the battle for Aleppo, amid fresh claims British intelligence is helping rebel fighters to oust the Syrian leader.

According to an opposition official, information on Syrian troop movements is helping rebels launch successful attacks on regime forces in the second city, where both sides have been locked in fierce fighting for weeks.

"British intelligence is observing things closely from Cyprus," the official told The Sunday Times.

"It's very useful because they find out a great deal.

"The British are giving the information to the Turks and the Americans and we are getting it from the Turks."

The newspaper quoted the official as saying British authorities "know about and approve 100%" intelligence from their Cyprus military bases being passed through Turkey to the rebel troops of the Free Syrian Army (FSA).

"The British monitor communications about movements of the government army and we got information about reinforcements being on their way to Aleppo," the official continued.

"We hit at the government troops in Idlib and Saraqib (southwest of Aleppo), with success."

It is the first indication that British intelligence is playing a covert role in the anti-regime revolt.

Aleppo - the country's commercial centre - has become the focus of the 17-month conflict, partly because of its strategic location near the Turkish border.

On Saturday, opposition fighters clashed with Syrian troops close to the city's airport.

Meanwhile, Syria denied reports that Mr Assad's deputy, vice-president Farouq al Shara, had defected.

More than 18,000 people have died in the bloodshed so far and around 170,000 have fled the country, according to the United Nations.