The Bush administration is considering taking the unprecedented step of preventing a visting head of state from addressing the United Nations in New York by denying a visa to Mahmoud Ahmadi-Nejad, Iran's new elected conservative president.

Officials said a decision rested on investigations into whether Mr Ahmadi-Nejad was involved in the 1979 US embassy hostage crisis and the killing of an Iranian-Kurdish dissident leader in Vienna in 1989. Iran denies his involvement in either event.