The message appeared on the secretary-general's main page and several internal ones
The United Nation's official website appears to have been hacked in a form of "cyber-protest" against Israeli and US policies in the Middle East.
A message appearing on the site early on Sunday said: "Hacked By kerem125 M0sted and Gsy That is CyberProtest Hey Ysrail and Usa dont kill children and other people Peace for ever No war."
Several web pages of the subsite for comments and news about Ban Ki-moon, the UN secretary-general, were affected and that section of the UN website was taken offline temporarily.
However, a placemarker graphic said the site was "unavailable due to scheduled maintenance".
The group of hackers, who have previously referred to themselves as "Turkish defacers", have been involved in thousands of website attacks, including a number of high-profile cases.
The US Air Force Reserve website was hit by what appears to be the same group about six months ago, displaying the message: "Hacked By_kerem125 Turkish Defacer and Muslım Solidear".
In July, the website for AAMI, one of Australia's largest insurance companies, was taken down after the messages "We are Turkish defacers" and "Hey Israel don't kill children" appeared instead of AAMI's website, according to The Sydney Morning Herald.