China and Russia have vetoed a draft resolution at the U.N. Security Council that pushed for a seven-day ceasefire in Syria's war-torn city of Aleppo.
Russia said the document has infringed the council rule allowing countries 24 hours to consider the final wording.
The claim was dismissed by the U.S. as a "made-up" alibi, saying Moscow wanted to preserve the recent military gains by Syrian government troops in Aleppo.
The army has reportedly seized about 70% of the rebel-controlled eastern portion of the city, according to a report from the BBC.
More than 100,000 people are said to be trapped in districts still under rebel control, where food supplies are exhausted and there are no functioning hospitals.
On Monday, Russia and China--both veto-wielding Security Council members--voted against the draft filed jointly by Egypt, New Zealand and Spain.
Venezuela also voted "No," while Angola abstained.
The remaining U.N. council members backed the resolution.
The document called for the ceasefire to allow the unimpeded access of aid to Aleppo.
Russia's U.N. Vitaly Churkin said the draft had not been given the standard 24 hours for analysis by council members.
The vote should have been postponed until a meeting of U.S. and Russian experts is convened later this week in Geneva, he added.
U.S. deputy envoy to the U.N., Michele Sison, accused Russia of making excuses.
"We will not let Russia string along the U.N. Security Council," she said.
French envoy Francois Delattre said Moscow is "decided to take Aleppo regardless of the human cost" of a military victory.
The U.K.'s representative Matthew Rycroft said the veto of Russia and its supporters "have held ransom the lives of hundreds of thousands of innocent, women, and children currently enduring hell in Aleppo."
This is the sixth time in five years that Russia has blocked a draft resolution on Syria by veto power.
Russia, a key ally of Syrian President Bashar al-Assad, has been carrying airstrikes against the rebels since Sept. 2015.
Aleppo was once the country's largest city and its commercial and industrial hub before the uprising against President Assad erupted in 2011.