When you step onto the grounds for the United Nations, you have not only left Manhattan, but you have left the United States as well. The United Nations is considered to be
International Territory owned by all of the nations who belong to the UN. However the surrounding neighborhoods are still in Manhattan.
This area is mainly made up of Tudor City, Beekman Place and Midtown East, with apartment buildings and townhouses for the residents, many of whom are
diplomats to the UN.
Turtle Bay/United Nations Guide