UNITED NATIONS (Reuters) - U.N. Secretary-General Ban Ki-moon condemned “preposterous” comments by a U.N.-appointed expert on Palestinian rights that there was a cover-up over the September 11 attacks, Ban’s chief of staff said on Monday.

The official, Vijay Nambiar, said however that it was not up to Ban to fire the expert, U.S. academic Richard Falk, as demanded by UN Watch, a Geneva-based advocacy group.

Falk wrote in a blog this month that there had been an “apparent cover-up” by U.S. authorities over the September 11, 2001 attacks, in which hijackers flew airliners into the World Trade Center in New York and the Pentagon near Washington.

He said mainstream media had been “unwilling to acknowledge the well-evidenced doubts about the official version of the events: an al Qaeda operation with no foreknowledge by government officials.”

In a letter to Ban last Thursday, UN Watch director Hillel Neuer called on the U.N. chief to “strongly condemn Mr. Falk’s offensive remarks -- and ... immediately remove him from his post.”

A letter of reply from Nambiar said Ban “condemns (Falk’s) remarks. He has repeatedly stated his view that any such suggestion is preposterous -- and an affront to the memory of the more than 3,000 people who died in the attack.”

Nambiar said Falk and other rights experts were not appointed by Ban but by the Geneva-based Human Rights Council, a 47-nation body created by the U.N. General Assembly in 2006. “Their continuance in their jobs is thus for the Council to decide,” he added.

UN Watch says on its website it is a non-governmental organization, accredited with the United Nations and affiliated with the American Jewish Committee, that aims to monitor U.N. performance against the yardstick of the U.N. Charter.

It supports U.N. goals but frequently criticizes the Human Rights Council, saying it constantly berates Israel but ignores many rights violations by developing countries. It has often targeted Falk, the council’s special rapporteur on human rights in the Palestinian territories, for anti-Israeli comments.

In a statement, Neuer welcomed Nambiar’s letter but said the Human Rights Council could not be trusted to fire Falk. He said Ban and U.N. human rights chief Navi Pillay had “the power and responsibility to play an influential and decisive role.”