UN chief Ban Ki-moon has said 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 on 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.
Secretary-General Ban warned on Saturday that 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 on Saturday that "the burden is on Russia… to salvage diplomatic efforts to restore a cessation of hostilities."
A week-long ceasefire agreed between the US 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.
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.