The United States set a March deadline on Thursday, November 29 for Iran to start cooperating in substance with a UN nuclear agency investigation, warning Tehran the issue may otherwise be referred to the UN Security Council, Reuters said.

The comments by U.S. diplomat Robert Wood to the board of the International Atomic Energy Agency signaled Washington's growing frustration at a lack of progress in the IAEA's inquiry into possible military dimensions to Tehran's nuclear program.

In his statement, Wood requested IAEA Director-General Yukiya Amano to say in his next quarterly report on Iran, likely due in late February, whether Tehran has taken "any substantive steps" to address the agency's concerns.

"If by March Iran has not begun substantive cooperation with the IAEA, the United States will work with other board members to pursue appropriate board action, and 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.

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

Amano earlier told the board that there had been no progress in his agency's year-long push to clarify concerns about suspected atom bomb research in Iran, but said he would continue his efforts.