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

Loyalist forces also battled rebels in Damascus after launching a ferocious assault to try to reinforce their 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 against using its chemical weapons in a conflict that shows no sign of ending after more than 17 months of bloodshed.

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 neighborhoods seized by the rebels at the weekend.

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

Rebels 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.

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.