The United States set a March deadline on Thursday for Iran [1] to start cooperating in substance with a U.N. nuclear agency investigation, warning Tehran the issue may otherwise be referred to the U.N. Security Council.

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.

The 27-nation European Union told the board that Iran's "procrastination" was unacceptable. "Iran must act now, in a substantive way, to address the serious and continuing international concerns on its nuclear program," it said.