More than 3,800 civilians have been killed in one year of Russian air strikes in Syria in support of President Bashar al-Assad, a monitoring group said Friday as international outcry mounted.

Assad's regime and its key backer Russia are under growing pressure from world governments to halt a new offensive pounding rebel-held areas of the battleground city of Aleppo.

More than 9,300 people have been killed in the Russian raids since September 30, 2015, the Syrian Observatory for Human Rights said.

The toll includes more than 2,700 jihadists from the Islamic State group and around 2,800 fighters from various rebel factions, the British-based monitor said.

At least 20,000 civilians have been wounded in the Russian raids, it said.

The Observatory -- which relies on a network of sources inside Syria for its information -- says it determines what planes carried out raids according to their type, location, flight patterns and the munitions involved.

Observatory director Rami Abdel Rahman said the death toll from Russian strikes could be even higher given the number of people killed by unidentified warplanes.

Moscow said on Thursday that it would press on with its bombing campaign in Syria, ignoring a threat by Washington to suspend its engagement over the conflict following escalating attacks on rebel-held parts of Aleppo.

Regime and Russian aircraft have carried out a barrage of strikes on east Aleppo since the Syrian government announced an offensive last week to retake all of the divided city.

- Merciless abyss -

The bombardment has been some of the worst in Syria's five-year civil war, and follows the failure of a short-lived ceasefire brokered by Russia and the United States.

Moscow and Washington have traded blame for last week's collapse of the ceasefire deal that would have marked the first step in a new effort to end the war that has killed 300,000 people since 2011.

US Secretary of State John Kerry admitted Thursday that months of diplomacy to end the war had hit a dead-end.

"I think we are on the verge of suspending the discussion because, you know, it's irrational in the context of the kind of bombing taking place, to be sitting there, trying to take things seriously," he said.

US President Barack Obama and German Chancellor Angela Merkel condemned what they called "barbarous" Russian and Syrian regime air strikes on Aleppo during a phone call, the White House said.

UN aid chief Stephen O'Brien told the Security Council in New York that Aleppo is descending into a "merciless abyss of a humanitarian catastrophe unlike any we have witnessed so far in Syria."

More than 100,000 children remain trapped in east Aleppo, he said.

Two of the largest hospitals in the city's east were bombed on Wednesday in what UN Secretary General Ban Ki-moon described as a war crime.

Save the Children said that bunker-busting bombs meant it was too dangerous for children to return to even underground schools in Aleppo when classes resume this weekend.

The "ferocious assault" on Aleppo could deprive almost 100,000 school-age children of an education, said the charity, which supports 13 schools in the city, eight of them underground.