Syrian government troops are continuing their large-scale offensive in Aleppo
Russia and China have vetoed a draft resolution at the UN Security Council that called for a seven-day ceasefire in Syria’s embattled city of Aleppo.
Russia said the document infringed the council rule allowing countries 24 hours to consider the final wording.
The US dismissed this as a "made-up alibi", saying Russia wanted to preserve recent military gains by Syrian government troops in Aleppo.
The army are reported to have seized more parts of rebel-held east Aleppo.
If confirmed, that would mean the government had recaptured about 70% of the rebel-controlled area in just over a week.
More than 100,000 people may be under siege 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 council members – voted against the draft submitted jointly by Egypt, New Zealand and Spain.
Venezuela also voted "No", while Angola abstained.
The other 11 UN Security Council members backed the resolution.
The document called for the ceasefire to allow the unimpeded access of aid to Aleppo.
Russia’s UN envoy Vitaly Churkin said the draft had not been given the traditional 24 hours for it to be analysed.
He added that the vote should have been postponed until a meeting of Russian and US experts on Tuesday or Wednesday in Geneva.
US deputy envoy to the UN, Michele Sison, accused Russia of using a "made-up alibi".
"We will not let Russia string along the Security Council," she added.
French envoy Francois Delattre accused Moscow of having "decided to take Aleppo regardless of the human cost" of a military victory,
The UK’s representative Matthew Rycroft said that in vetoing the resolution Russia and its supporters "have also held to ransom the lives of hundreds of thousands of innocent men, women and children currently enduring hell in Aleppo".
This is the sixth time in five years that Russia has used its veto power to block a draft resolution on Syria.
Russia, a key ally of Syria’s President Bashar al-Assad, has been carrying out air strikes against rebels since September 2015.
Aleppo was once Syria’s largest city and its commercial and industrial hub before the uprising against President Assad began in 2011.