Ban condemned Zawahiri's statement in a morning get-together with UN staff in Romania's capital Bucharest, where he is attending a three-day NATO summit that is discussing the situation in Afghanistan.

"The Secretary General just told UN staff in Romania that the Al-Qaeda statement was 'totally false and unacceptable'," a UN spokesman travelling with Ban said

The spokesman added that Ban had earlier held discussions with Afghan President Hamid Karzai on the United Nations' support for the Organisation of the Islamic Conference and the Islamic world.

"The Secretary General was very clear today that United Nations should be seen as a friend of the Muslim world and that he was alarmed by the Al-Qaeda accusations," he said.

In an online audiofile the day before, Zawahiri had said, "The United Nations is an enemy of Islam and Muslims: it is the one which codified and legitimised the setting up of the state of Israel and its taking over of the Muslims' lands."