Stephen O'Brien, the UN under-secretary-general for humanitarian affairs, told the Security Council that the use of siege and starvation as a weapon of war was "reprehensible" and "must stop immediately."
The new figure, up from 517,700, marks a further increase from a previous estimate of 486,700 people living in besieged areas.
It includes the residents of the Al Wa'er area of Homs in western Syria that have been under siege by Syrian government forces since March, said O'Brien.
"Today 592,700 people are living in besieged areas because of the appalling, deteriorating situation in Al Wa'er," he told the council.
The United Nations is preparing to begin humanitarian air drops over besieged areas starting June 1, after its repeated demands for access to the blockaded towns were refused.
In May, Syrian authorities allowed aid deliveries to 14 hard-to-reach areas, far short of the requests to reach 35 towns on the UN list.
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O'Brien said the Damascus regime had denied aid to more than 40 percent of the target population for May, including in Aleppo, Al Wa'er and Talbiseh.
Syrian Ambassador Bashar Jaafari denied that his government was blocking aid and said that 19 of the 26 UN requests for humanitarian convoys had been approved for the month of May.
Of the 19 approved convoys, the United Nations only sent three, said Jaafari.
The ambassador complained that the situation concerning humanitarian aid "is manipulated" and that UN figures were "far from reality, based on unknown sources or unreliable ones."
According to the United Nations, 452,700 Syrians are living in towns besieged by regime forces, while 110,000 are under siege by the Islamic State group in the eastern city of Deir Ezzor.
Some 20,000 are under siege by rebel groups in Foah and Kefraya in Idlib province and 10,000 are surrounded by both Syrian forces and rebel groups in Yarmouk in Damascus.
Boosting humanitarian aid is a key condition to be met before political talks on ending the war can resume.
The talks are aimed at reaching a settlement to end the five-year war that has left 280,000 dead and driven millions from their homes.