Amman: Syrian army shells crashed into southern Damascus on Wednesday and helicopters fired rockets and machineguns during an assault to shore up President Bashar Al Assad’s grip on the capital, opposition activists said.

They said at least 40 people had been killed in what they called the heaviest bombardment this month.

“The whole of Damascus is shaking with the sound of shelling,” said a woman in Kfar Souseh, one of several districts hit during the military offensive to root out rebel fighters. At least 22 people were killed in Kfar Souseh and 18 in the nearby district of Nahr Eisha, activists said.

“There are 22 tanks in Kfar Souseh now and behind each one there are at least 30 soldiers. They are raiding houses and executing men,” an opposition activist in Kfar Souseh, who gave his name only as Bassam, told Reuters by Skype.

More than 250 people, including 171 civilians, were killed across Syria on Tuesday, mostly around Damascus, Aleppo and the southern city of Dera’a, according to the Syrian Observatory for Human Rights, a British-based opposition monitoring group.

Fighting a 17-month old revolt against Al Assad’s rule, the army has used tanks and helicopter gunships this week in an offensive around the capital which has coincided with the departure of UN military observers after a failed mission.

Activists in the southwestern suburb of Mouadamiya said regime’s forces had killed 86 people there since Monday, half of them in cold blood. Amidst the violence, Russia’s Foreign Minister Sergei Lavrov said Tuesday that the Syrian government is preparing for an immediate political dialogue with the opposition. As Syria slips deeper into chaos, the United States and Israel have voiced concern that Assad might lose control of his chemical weapons arsenal or even be tempted to use it. Russia, a Syrian ally since Soviet times, believes Syria has no intention of using its chemical weapons and is able to safeguard them, the Russian newspaper Kommersant reported on Wednesday, citing an unidentified Foreign Ministry official.

US President Barack Obama threatened Assad on Monday with “enormous consequences” if he employed chemical weapons or even if he moved them in a menacing way, drawing a warning from Russia against any unilateral action by the West. Meanwhile, France said Wednesday that it was providing the Syrian opposition with “non-lethal” military aid, Prime Minister Jean-Marc Ayrault said Wednesday.