|Syrian civil defence volunteers, known as the White Helmets, carry the body of a victim following a government forces air strike in Aleppo on October 4, 2016|
United Nations humanitarian agency (OCHA) spokesman Jens Laerke said eastern Aleppo now met all three criteria used to define an area as besieged.
That includes military encirclement, lack of humanitarian access and the lack of free movement for civilians.
The UN estimates that there are 275,000 people in eastern Aleppo under siege, Laerke said. The west of the city is controlled by the government and has continued to receive relief supplies.
Once Syria's economic powerhouse, Aleppo has been devastated by the country's brutal five-year civil war, with the suffering intensifying since regime troops cut off the last supply route in July.
The UN and Red Cross have been pushing to get aid into east Aleppo for weeks, but those efforts have been stalled by insecurity and bureaucratic problems, including obstacles imposed by both the Damascus government and rebel commanders.
After being formally declared besieged in OCHA's latest report to the UN Security Council, eastern Aleppo will "be included automatically in the monthly plans for access either through cross-border deliveries or cross-line deliveries," Laerke said in an email.
There are now 18 besieged areas in Syria, according to the UN.
Eastern Aleppo has replaced Daraya on the list, after the latter town was emptied of residents and opposition fighters under a deal with the government.