VIENNA - The United States effectively set a March deadline on Thursday for Iran to start cooperating in substance with a UN nuclear agency investigation, saying it would otherwise urge reporting the issue to the UN Security Council.

The comments by US diplomat Robert Wood to the 35-nation governing board of the UN International Atomic Energy Agency signaled Washington's growing frustration at the lack of results in the IAEA's inquiry into possible military dimensions to Tehran's nuclear program. Iran denies the charge.

"If by March Iran has not begun substantive cooperation with the IAEA, the United States ... would urge the board to consider reporting this lack of progress to the UN Security Council," Wood said, according to a copy of his statement.

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"Iran cannot be allowed to indefinitely ignore its obligations ... Iran must act now, in substance," Wood said.

IAEA chief Yukiya Amano earlier on Thursday said the UN agency had made no progress in a year-long push to find out if Iran worked on developing an atomic bomb, despite "intensive efforts" by his agency.

"No concrete results have been achieved," Yukiya Amano, director general of the International Atomic Energy Agency (IAEA), told a quarterly meeting of the 35-nation governing board of the United Nations' nuclear watchdog.

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