Almost 150 civilians have been killed in a week of intense violence in the besieged eastern half of Aleppo, activists said as violence continued to grip Syria’s former industrial capital.

The latest casualty figures cap two months of unprecedented violence in Syria’s largest city. More than 800 people have been killed since forces loyal to the regime of Bashar al-Assad announced a campaign to crush the opposition in the rebel-held eastern districts.

“This ferocious campaign is a war of extermination,” said a doctor in eastern Aleppo, who was wounded earlier this month in an airstrike. “Everything is a target, whether human or tree or rock. Everything is being exterminated with the collusion of the United Nations. They all see and hear, but they will not answer, and they cannot stop this war machine.”

He added: “We have nobody but God,.”

The Syrian Observatory for Human Rights, a UK-based organisation, said on Tuesday that 141 civilians, including 18 children, had been killed in the last week of violence.

Moscow intervened last year in the conflict to shore up the Assad regime. Observers had predicted that the renewed assault on Aleppo would begin with the highly publicised arrival a week ago of an aircraft carrier belonging to Russia off the Syrian coast.

Those fears have been realised one week into the vaunted campaign, with relentless aerial bombardment that has reduced to rubble civilian homes and left the rebel-held east without any functioning hospitals for a quarter of a million people, including almost 100,000 children, according to UN estimates, who are still living there. The city has been besieged since July.

“I am more or less at my wits’ end,” said Stephen O’Brien, the UN’s top humanitarian official, at a security council briefing on Monday, in a blistering condemnation of the international community’s paralysis. “Shame on us all for not acting to stop the annihilation of eastern Aleppo and its people and much of the rest of Syria too.”

The Observatory said on Tuesday that 834 civilians, including 176 children, had been killed over the past two months.

Casualty figures over the past three days have been difficult to come by because they are usually tracked by local hospitals, which have been put out of service by bombing.

Aleppo has been divided since 2012, with the west under government control and the east held by rebels. Its fate has long been seen as a bellwether for the momentum of the Syrian war, and forces loyal to Assad hope to deal a fatal blow to the rebellion by seizing the last major urban stronghold under opposition control.

Loyalist forces include thousands of Iranian-backed militias along with a contingent of Syrian government troops who have so far made limited incursions into the rebel-held east. That battle will likely result in numerous casualties and intense urban combat that would leave the neighbourhoods still standing in ruins.