Russia and China on December 5 vetoed a United Nations Security Council resolution pushed for by Western powers and calling for a seven-day truce in the Syrian city of Aleppo.
Russia argued that the measure would allow rebel forces to regroup and should be postponed pending talks between Washington and Moscow scheduled for this week.
It was the sixth time Russia has vetoed a Security Council resolution on Syria since the conflict started in 2011 and the fifth time China has blocked action.
"These kinds of pauses have been used by fighters to reinforce their ammunition and to strengthen their positions and this will only worsen the suffering of civilians," Russia's UN Ambassador Vitaly Churkin said.
Russian Foreign Ministers Sergei Lavrov said earlier in the day that Moscow would start talks with Washington this week on the withdrawal of opposition forces from Aleppo following significant gains made by Russian-backed Syrian forces in recapturing rebel-held areas since mid-November.
"The concrete route and time frame for the withdrawal of all fighters from eastern Aleppo will be agreed upon," Lavrov said. "As soon as these routes and time frames are agreed on, a cease-fire can come into effect."
At the UN, Churkin argued that action by the Security Council should be postponed to allow time for those negotiations.
But U.S. officials said no agreement on withdrawal is near. Rebel forces have rejected calls for them to evacuate.
"That is a made-up alibi," Deputy U.S. Ambassador to the United Nations Michele Sison told the council.
"We have not reached a breakthrough because Russia has been more focused on preserving its military gains than helping Aleppo's citizens," she said. "We will not let Russia string along this Security Council while waiting for a compromise from the Russians that never seems to come."
Sison said the veto of the cease-fire resolution was "a death sentence for innocent men, women and children" in Aleppo.
The United Nations says more than 200,000 civilians might still be trapped in rebel-held areas of Aleppo, which are experiencing severe food and aid shortages.
British UN Ambassador Matthew Rycroft expressed surprise that China joined Russia in vetoing the resolution, which would have allowed humanitarian aid access while quelling violence throughout Syria by both rebel forces and Syrian President Bashar al-Assad's troops.
Rycroft concluded Russia and China blocked the measure "because of their longstanding, misplaced faith in a despot who has killed nearly half a million of his own people."
Chinese UN Ambassador Liu Jieyi said the vote on the draft text should have been delayed to allow for more negotiations to reach a consensus on the council. He accused Rycroft of "poisoning" the atmosphere and "abusing" the forum with his remarks.
Had the resolution been adopted, it would have been a "fragile glimpse of hope" and allowed the UN to "save lives," French ambassador Francois Delattre said.
He accused Russia of having "decided to take Aleppo regardless of the human cost."
New Zealand UN Ambassador Gerard van Bohemen, who helped draft the resolution, said the failure to act was "deeply damaging to the council's reputation and catastrophic for the people of Syria."