The Syrian army recaptured three Christian neighbourhoods in the historic heart of Aleppo from rebels Thursday but fierce clashes continued in other parts of the city, residents said.

Loyalist forces also battled rebels in Damascus, shelling districts and storming houses after launching a ferocious assault to try to reinforce its hold on the capital, activists said.

The West stepped up the pressure on President Bashar al-Assad's government, with Britain joining the United States in warning it not to use its chemical weapons.

Washington is also ramping up its military presence in the region, dispatching a nuclear-powered aircraft carrier to deal with "threats" from Iran and the turmoil in Syria.

Aleppo residents spoke of heavy exchanges in the heart of the city during the army's offensive to recapture neighbourhoods seized by the rebels at the weekend.

"We have had the worst two days of our lives," said Sonia, the wife of a wealthy businessman in the northern city told AFP.

"If our house weren't built like a fortress, we'd all be dead. The entrance is very badly damaged."

Rebels had seized control of three Christian quarters of the Old City of Aleppo during the weekend, including Jdeide and Telal, once frequented by tourists for their restaurants and handicraft shops.

The rebel Free Syrian Army had also seized the nearby neighborhood of Sulamaniyeh, most of whose inhabitants are Armenian Christians.

The neighborhood is home to some ancient monasteries and a cathedral of the Melkite Greek Catholic Church.

"Battles on Monday and Tuesday were very violent, and they lasted for many long hours before the army managed to expel the rebels," a resident of the neighborhood told AFP by telephone.

"After cleansing the area, hundreds of residents of the districts of Telal and Sulamaniyeh took to the streets to celebrate and express their support for the army," he added.

After the rebels' expulsion, residents set up "popular committees" to prevent their return, another resident told AFP.

Communities like those in the heart of Aleppo are among those in which support for Assad's government and suspicion of the rebels runs highest.

But in the southern belt of Damascus, where fighting also raged on Thursday, opposition to the the government runs high.

"Parts of Damascus look like Gaza, with the army deployed on the outside, setting up major checkpoints, but unable to get in," said a Damascus resident and opposition activist who identified herself as Samara.

"Fear is everywhere," Samara told AFP via Skype. "Most people in the violence-stricken areas are stuck in their homes."

The army shelled the south Damascus district of Al-Hajar Al-Aswad, scene of bitter fighting last month, as well as the town of Daraya, on the capital's outskirts, for a second day in a row, the Syrian Observatory for Human Rights said.

"The situation in Daraya is tragic," said Abu Kinan, an activist based in the suburb of sonme 200,000 people. "I am alive now, but they are shelling non-stop. Death is everywhere."

At least 41 people were reported killed in Damascus on Wednesday alone after troops backed by combat helicopters and tanks attacked several areas in the southern belt where anti-regime sentiment is strong, the Observatory said.

It was some of the worst violence since the army claimed to have reclaimed most of the capital a month ago.

"Regime forces are carrying out summary executions and destroying the homes of their opponents in a bid to crush the revolt once and for all," said Observatory director Rami Abdel Rahman.

An activist who gave his name as Omar told AFP from the Damascus district of Qaboon, which was the scene of heavy fighting on Wednesday, that the army and the rebels were playing a game of "cat and mouse."

"The army's attempts cannot succeed, because they cannot get into the neighbourhoods where the Free Syrian Army are hiding," he said.

Turkish and U.S. officials began their first "operational planning" meeting aimed at hastening the end of Assad's embattled regime. Turkey has given shelter to the rebel leadership as well as tens of thousands of civilian refugees.

Turkish foreign ministry deputy under-secretary Halit Cevik and U.S. ambassador Elisabeth Jones led the delegations made up of intelligence agents, military officials and diplomats at the Ankara talks, a foreign ministry source told AFP.

The officials were to discuss contingency plans in the case of potential threats including a chemical attack by Assad's regime which Washington has said would be a "red line."

British Prime Minster David Cameron and U.S. President Barack Obama warned after a Wednesday telephone call that they would be forced to consider a new course of action if Syria threatened to use chemical weapons against rebel fighters.

The two leaders agreed that the use of chemical weapons -- which Syria admitted possessing in July -- would be "completely unacceptable."

U.S. Defence Secretary Leon Panetta said the United States was preparing for "threats" from Iran and Syria's turmoil as an aircraft carrier headed to the region ahead of schedule.

The Observatory said that nearly 25,000 people have now been killed in Syria since March 2011. The UN puts the death toll at around 17,000.