U.S. officials, meanwhile, confirmed the Obama administration has had direct contact with Iran over the allegations. The U.S. ambassador to the United Nations, Susan Rice, met with Iranian officials at Iran's mission to the U.N. on Wednesday — a highly unusual contact for two countries that do not have diplomatic relations.

Obama would not say whether Iran's supreme leader, Ayatollah Ali Khamenei, or its president, Mahmoud Ahmadinejad, knew of the alleged plan. Yet he called it part of a pattern of "dangerous and reckless behavior" by the Iranian government and said people within that government were aware of a murder-for-hire plot.

The U.S. considers it an attempted act of terrorism.