UN chief Ban Ki-moon said Saturday he was "appalled by the chilling military escalation" in Syria's battleground city of Aleppo where residents cowered indoors as air strikes toppled buildings and killed at least 45 civilians.
The Security Council was set to meet Sunday to discuss the upsurge in violence since the Syrian army announced an offensive to retake the rebel-held east of the devastated city.
Nearly two million civilians were left without water in Aleppo after regime bombardment damaged a pumping station and rebels shut down another in retaliation, the UN said.
Ban warned the use of bunker buster bombs and other advanced munitions against civilians may amount to war crimes, after the army Thursday launched the offensive backed by Russian air raids that has cost around 100 lives.
Top EU officials said that the attacks on civilians amount to a "breach of international humanitarian law," and called for intensified peace efforts.
Washington and leading European powers said Saturday that "the burden is on Russia... to salvage diplomatic efforts to restore a cessation of hostilities."
A week-long ceasefire agreed between the United States and Russia ended on Monday and efforts to revive the truce failed.
That same day, an aid convoy was hit by an air strike that US officials have said was carried out by Russian planes, although Moscow has denied responsibility.
- 'Cease bombing women and children' -
US Secretary of State John Kerry, who failed in talks with Russian counterpart Sergei Lavrov to revive the ceasefire, earlier said: "What is happening in Aleppo today is unacceptable. It is beyond the pale."
"If people are serious about wanting a peaceful outcome... they should cease and desist bombing innocent women and children, cease cutting off water and laying siege in mediaeval terms to an entire community," he said.
Kerry had harsh words for Moscow's involvement in the conflict, in comments at a meeting with his European counterparts.
"Russia needs to set an example, not a precedent -– an unacceptable precedent, I might add, for the entire world," he said.
Syria's regime said it was confident of victory, with Foreign Minister Walid Muallem telling the UN General Assembly that the army and its allies were making "great strides" in the conflict.
He said a US-led coalition air strike that killed at least 62 Syrian soldiers on September 17 was intentional "and not an error, even if the United States claims otherwise".
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Rebel-held eastern districts of Aleppo came under intense air and artillery fire for a fifth night on Friday ahead of an anticipated ground offensive by the army to recapture the whole of the divided city.
Muallem told the UN: "Our belief in victory is even greater now that the Syrian Arab Army is making great strides in its war against terrorism, with the support of the true friends of the Syrian people," singling out Russia, Iran and Lebanon's Shiite militant group Hezbollah.
Saturday's death toll of 45 in Aleppo city was expected to rise because people remained trapped in the rubble, said the Syrian Observatory for Human Rights, a Britain-based monitoring group.
It said another seven people were killed elsewhere in Aleppo province on Saturday.
"We were home when a missile crashed into our road," said one resident of the Bab al-Nayrab district who gave his name as Nizar.
"Half of the building just caved in and our baby was hit on the head. He died on the spot," Nizar said, the body of his son on the ground wrapped in a blanket.
- Massive destruction -
Seven people were killed in a strike as they queued to buy yoghurt at a market in the Bustan al-Qasr district on the front line dividing the government-held west from the rebel-held east of the city.
The attack left a pool of blood and body parts strewn across the site, said an AFP correspondent on the scene.
Medics said they were carrying out many amputations to try to save the wounded, while supplies of blood and IV drips were running out.
On Friday, at least 47 people were killed in heavy bombing, among them seven children, the Observatory said.
There was massive destruction in several neighbourhoods, including Al-Kalasseh and Bustan al-Qasr, where some streets were almost erased by the bombardment.
Residents and activists said one type of bomb had produced earthquake-like tremors upon impact, razing buildings right down to their basements where many residents desperately seek safety during attacks.
Further south in the central city of Homs, a convoy of 36 aid trucks reached the rebel-held district of Waer, the International Committee of the Red Cross said.
The denial of access to food, water and medicines has been used repeatedly as a weapon by all sides in the five-year war which has cost more than 300,000 lives and displaced over half the population.
The approximately 250,000 people in east Aleppo have been under near-continuous siege since government troops encircled the area in mid-July.