The new figure of 974,080 people marks a dramatic increase from 486,700 Syrians living in besieged areas just six months ago, Stephen O'Brien told the Security Council.
"Nearly one million Syrians are living tonight under siege," O'Brien said.
"Civilians are being isolated, starved, bombed, denied medical attention and humanitarian assistance in order to force them to submit or flee."
Some of the areas added to the UN's siege list are located in the Eastern Ghouta region of rural Damascus.
Condemning this "deliberate tactic of cruelty," O'Brien said the sieges were mostly perpetrated by Syrian government forces against civilians.
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O'Brien, the under-secretary-general for humanitarian affairs, renewed his call for an end to besiegement.
The council was meeting to discuss the crisis in Syria as Syrian and Russian warplanes pounded rebel-held parts of northern Syria including Aleppo, where food rations were running out.
"The situation is horrific, catastrophic," said French Ambassador Francois Delattre who accused the Damascus government of waging a "total-war strategy to take back Aleppo, no matter the price."
Delattre said the strategy would fail, pushing more Syrians to join the Islamic State group and fueling the terrorism that the Damascus government maintains it is combating.
British Ambassador Matthew Rycroft said the Russian-backed Syrian bombing of Aleppo was "barbaric" and called on Moscow and Damascus to stop.