WASHINGTON – President Obama said Thursday that officials at the “highest levels” of the Iranian government must be held accountable for a brazen and bizarre plot to assassinate the Saudi ambassador to the United States on American soil, insisting world leaders will believe the U.S. case once they absorb the details.

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.

“We believe that even if at the highest levels there was not detailed operational knowledge, there has to be accountability with respect to anybody in the Iranian government engaging in this kind of activity,” Obama said in a news conference tied to the state visit of South Korean President Lee Myung-bak.

Iran has vehemently denied anything to do with the alleged plot to kill the Saudi envoy to the United States, Adel Al-Jubeir, at a Washington restaurant.

U.S. officials have described it as a remarkably clumsy but deadly serious operation by Iran’s elite foreign action unit, the Quds Force. Two men were charged in New York federal court Tuesday for allegedly trying to hire a purported Mexican drug cartel member to carry out the assassination with a bomb attack.

Obama is seeking a vigorous response from around the world, on top of increased sanctions and pressure against Iran from the United States itself.

U.S. diplomats have given their host governments information about the foiled plot.