As Syrian regime forces advanced in Aleppo, backed by a deadly Russian air campaign, Doctors Without Borders (MSF) decried the "bloodbath", saying the battleground city's eastern rebel-held portion had become "a giant kill box".
"Bombs are raining from Syria-led coalition planes and the whole of east Aleppo has become a giant kill box," MSF director of operations Xisco Villalonga said in a statement on Friday.
"The Syrian government must stop the indiscriminate bombing, and Russia as an indispensable political and military ally of Syria has the responsibility to exert the pressure to stop this," he said.
Syria's army was advancing on two Aleppo fronts, as talks between key players Washington and Moscow - which back opposing sides in the war - appeared close to collapse.
Damascus's bid to recapture all of the divided northern city has prompted the UN to warn of "a humanitarian catastrophe" and Aleppo is the focus of a new French-led UN draft resolution calling for the cessation of hostilities.
Just over a week after Syria's army announced an operation to recapture all Aleppo, it was advancing both in northern and central Aleppo on Friday, according to the Syrian Observatory for Human Rights monitor and state media.
In the north, it recaptured the Handarat former Palestinian refugee camp, as well as the old Kindi hospital, said Observatory director Rami Abdel Rahman.
Rebels had held the hospital since 2013, and capturing it allows government forces to threaten the opposition-held Heluk and Haydariyeh neighbourhoods.
The Observatory said at least 15 people, including two children, were killed in strikes on Heluk and other eastern districts Friday.
In leaked audio published by The New York Times, US Secretary of State John Kerry expressed frustration that his diplomatic efforts to end Syria's civil war had not been backed up by US military force.
"I've argued for the use of force ... but things evolved into a different process," he said to a group of Syrian civilians last week in New York.
UN chief Ban Ki-moon also announced he was setting up an internal board of inquiry to investigate the 19 September bombing of an aid convoy in Syria that killed 18 people.
The US has blamed the bombing on Moscow, which denies the accusation.
In central Aleppo, meanwhile, fierce clashes shook the Suleiman al-Halabi neighbourhood, divided by the frontline separating the rebel-held east and regime-held west.
The army is seeking to capture the opposition-held sector of the district and advance to the main water supply station for the government-controlled part of Aleppo which is in the neighbourhood.
State television said 15 civilians had been killed and 40 wounded by rocket fire into the government-held part of Suleiman al-Halabi and neighbouring Midan district.
Since the army operation began, Damascus and Moscow have pounded east Aleppo with air strikes, barrel bomb attacks and artillery fire, killing at least 216 people, including more than 40 children, according to the Observatory.
The assault has levelled apartment blocks and put hospitals out of service, creating a humanitarian catastrophe in opposition areas besieged for most of the past two months.
It has been some of the worst violence since the March 2011 beginning of Syria's conflict, which has killed more than 300,000 people and displaced over half the population.
Outside Damascus, meanwhile, air raids on several rebel-held towns in the Eastern Ghouta region killed at least 17 people including eight children, the Observatory said.
Moscow, a key ally of President Bashar al-Assad, began a military campaign to bolster his forces in September 2015 that has so far killed more than 9,300 people, the Britain-based Observatory says.
That figure includes 3,804 civilians and more than 5,500 jihadists and rebels, it says, adding that at least 20,000 civilians have been wounded.
The Observatory says it determines what planes carried out raids according to their type, location, flight patterns and the munitions involved.
"We do not consider as reliable the information ... coming from this organisation, which is based in the United Kingdom," Kremlin spokesman Dmitry Peskov told reporters.
Moscow said Thursday it would continue its campaign, despite Washington's threat and international concern about Aleppo.
British Foreign Secretary Boris Johnson said Russia was in danger of becoming "a pariah nation", saying the attacks in Aleppo were "unquestionably a war crime".