The United States has called the assault on Aleppo by Syria and Russia "a gift" to Islamic State, saying it was sowing doom and would generate more recruits for the militant group.
Moscow vowed to press on with its offensive in Syria, while US officials searched for a tougher response to Russia's decision to ignore the peace process and seek a military victory on behalf of Syrian President Bashar al-Assad.
United Nations aid chief Stephen O'Brien urged the 15-member UN Security Council on Thursday to stop "tolerating the utter disregard for the most basic provisions of international humanitarian law."
The recent focus of the fighting is a Syrian and Russian bid to recapture rebel-held eastern Aleppo.
"East Aleppo this minute is not at the edge of the precipice, it is well into its terrible descent into the pitiless and merciless abyss of a humanitarian catastrophe unlike any we have witnessed in Syria," O'Brien said.
"The only remaining deterrent it seems is that there will be real accountability in the court of world opinion and disgust - goodness knows, nothing else seems to be working to stop this deliberate, gratuitous carnage of lives lost," he said.
Syria's UN Ambassador Bashar Ja'afari rejected accusations on Thursday that the Syrian government was killing civilians.
"The Syrian government is not bombing civilians. These people are our own people. We don't bomb civilians, we don't kill civilians. We don't bomb humanitarian convoys. We don't do that. Those who did it are the terrorists," Ja'afari told reporters after a Security Council meeting.
French UN Ambassador Francois Delattre said he had started discussions with some council members on a draft resolution to try and impose a ceasefire in Aleppo.
New Zealand UN Ambassador Gerard van Bohemen, president of the council for September, said after the council meeting that members expressed a "great deal of interest" in seeing the French text and a willingness to work on it.
US Ambassador Samantha Power said that what Assad and Russia were doing in Aleppo was "soul-shattering."
"What they are doing is sowing not only the doom of this country ... but it is going to generate more refugee flow, more radicalisation.
"What they are doing is a gift to ISIL (Islamic State) and (Nusra Front), the groups that they claim that they want to stop," she said.
Russia has also accused the United States of "de facto support for terrorism" in Syria.
As Washington threatens to walk away from talks with Russia on Syria unless the fighting stops, Britain's UN Ambassador Matthew Rycroft told reporters it was time "to move to a different form of diplomacy," pointing to the Security Council.
Rycroft also dismissed a Russian proposal for a 48-hour humanitarian pause in fighting in Aleppo.
Since July, the UN has been calling for a weekly 48-hour truce to allow the delivery of aid to besieged areas.
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