The United States has established an ultimatum to Iran that it must substantively cooperate with the United Nations nuclear watchdog by March otherwise Washington will take the matter up with the U.N. Security Council.

Iran has thus far refused to cooperate with the International Atomic Energy Agency’s probe into the military aspects of its nuclear program, citing it seeks to develop atomic power purely for peaceful purposes. Both the U.S. and Israel suspect Teheran is building nuclear bombs.

"Iran cannot be allowed to indefinitely ignore its obligations. ... Iran must act now, in substance," U.S. diplomat Robert Wood told the IAEA board.

Reuters reported that IAEA chief Yukiya Amano admitted that his agency has failed to uncover the true nature of Iran’s nuclear ambitions. But he vowed to carry on his investigation.

"Now is the time for all of us to work with a sense of urgency and seize the opportunity for a diplomatic solution," Amano said in Vienna.

"The IAEA is firmly committed to dialogue.”

But the IAEA warned last week that Iranian nuclear officials are preparing to double production at its Fordo atomic facility, enriching uranium to purities of 20 percent, which is almost sufficient to manufacture a bomb.

Last week, the P5+1 powers -- the U.S., China, Russia, Britain, France and Germany -- met in Brussels and declared they want to renew nuclear negotiations with Iran "as soon as possible."