Syria's regime intensified air strikes on rebel-held areas of Aleppo province Monday as a UN envoy prepared to meet US and Russian officials to try to revive peace talks.
France called for an immediate humanitarian truce in Aleppo city after civilian deaths in the provincial capital and after four hospitals were bombed there on Sunday.
On Monday, air strikes and barrel bombs killed 22 civilians in rebel-held parts of the northern province, and rebel rocket fire on government areas killed three more, a monitor said.
The bombardment came as UN Syria envoy Staffan de Mistura was expected to meet top US and Russian officials in Geneva on Tuesday with the aim of reviving peace talks to end the five-year conflict.
French Ambassador to the UN Francois Delattre compared Aleppo's plight to that of Sarajevo during the Bosnian war, and said "the Security Council simply cannot accept such war crimes -- yes war crimes -- to repeat again".
Syria and its allies are "determined to besiege, starve and bomb Aleppo until they reach their military goal -- eradicating the opposition -- this is what the siege is all about," Delattre said.
Britain's UN Ambassador Matthew Rycroft said: Enough is enough now."
More than 280,000 people have been killed in Syria and more than half its population has been displaced since the conflict began with the brutal repression of anti-government protests in 2011.
US mission spokesman Paul Patin said Washington's special envoy for Syria Michael Ratney would be at Tuesday's meeting in Geneva.
Russia's Ria-Novosti news agency said deputy foreign minister Gennady Gatilov would represent Moscow.
US Secretary of State John Kerry and his Russian counterpart Sergei Lavrov were also due to meet Tuesday on the sidelines of an Asian summit in Vientiane, Laos.
Earlier this month, the two announced an agreement on "concrete steps" to salvage a failing nationwide ceasefire in Syria, a key step before negotiations can resume.
The February ceasefire between the regime and non-jihadist rebels -- which was brokered by the US and Russia -- remains largely in tatters.
The Britain-based Syrian Observatory for Human Rights said 10 civilians were killed in air strikes on the rebel-held town of Atareb in Aleppo early on Monday.
It said the strikes, believed to have been carried out by Russian warplanes, hit several locations including a market area.
Russia is a key ally of President Bashar al-Assad and began air strikes in support of regime forces in September 2015.
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.
Footage obtained by AFP of the aftermath of the strikes showed the local civil defence unit trying to put out a fire in the rubble of one collapsed building.
Nearby, people combed through debris trying to extract medicine from what appeared to be the remains of a pharmacy or clinic.
Elsewhere in Atareb, crushed fruit and vegetables were mixed in with pieces of broken brick and shattered concrete.
In Aleppo city itself, at least 12 civilians were killed in barrel bomb attacks by government forces on eastern rebel-held neighbourhoods, the Observatory said.
An AFP journalist at the scene of one of the attacks in the Al-Mashhad district earlier said civil defence workers were struggling to retrieve survivors who were still trapped under heavy debris.
He said rescuers had managed to pull one boy alive from the rubble, but the rest of his family were dead and still trapped under a collapsed building.
Once Syria's economic powerhouse, Aleppo has been roughly divided between government control in the west and rebel control in the east since mid-2012.
In recent weeks, government forces seized parts of the only remaining supply route into the city's east, cutting off opposition districts from the outside world and prompting food shortages and spiralling prices.
The government regularly bombs the east of the city, and rebels often fire rockets into western districts.
The Observatory said at least three civilians were killed by rebel fire on western Aleppo on Monday.
Syrian state news agency SANA said a pregnant woman and her child were killed.
SANA also reported that four people had been wounded on Monday in rebel rocket fire on central Damascus.
Rebel rockets also hit Old Damascus late Sunday after air raids struck four makeshift hospitals in Aleppo's rebel districts, jeopardising medical care for more than 200,000 civilians besieged there.