On Sunday, US permanent representative Samantha Power said Russian and Syrian forces were “lying waste” to besieged rebel-held areas of the city of Aleppo.
Kremlin spokesman Dmitry Peskov warned that such language might damage efforts to end the five-year civil war.
Activists meanwhile reported dozens of fresh air strikes on Aleppo overnight.
The Syrian Observatory for Human Rights, a UK-based monitoring group, said at least two civilians had been killed and many wounded.
At least 128 people are believed to have died in rebel-held Aleppo since last week, when a truce brokered by the US and Russia collapsed.
The US, UK and France, which back the opposition to Syrian President Bashar al-Assad, openly accused Russia of lying about its involvement in the assault on Aleppo at an emergency session of the UN Security Council.
British permanent representative Matthew Rycroft said Mr. Assad’s forces and Moscow had “plunged to new depths and unleashed a new hell on Aleppo”, which was once Syria’s largest city and the country’s industrial and commercial hub.
He alleged that they were committing war crimes by using bunker-busting bombs to destroy underground shelters, dropping incendiary weapons indiscriminately on civilian areas, and targeting the city’s water supplies.
Ms. Power told the meeting: “Instead of pursuing peace, Russia and Assad make war. Instead of helping get life-saving aid to civilians, Russia and Assad are bombing the humanitarian convoys, hospitals, and first responders who are trying desperately to keep people alive.”
“What Russia is sponsoring and doing is not counterterrorism; it is barbarism,” she added.
Pro-Kremlin media in Russia have been gripped by the discussions on Syria at the UN Security Council, but they offered little comment on the accusations levelled against Moscow.