A new UN report shows that Iran "willfully" withheld information about its effort to develop technology that could lead to the building to nuclear weapons, the US said on Tuesday.
"The Iranians have been wilfully non-cooperative. And you can read that in the report," US State Department spokesman Sean McCormack said. "It's disturbing."
The UN nuclear watchdog International Atomic Energy Agency (IAEA) released a report Monday saying Iran has failed to adequately answer questions based on intelligence that shows the Islamic state may have sought nuclear arms.
"Substantive explanations are required from Iran to support its statements on the alleged studies and on other information with a possible military dimension" of its nuclear activities, the IAEA report said.
Iran has a civilian nuclear energy programme but rejects allegations from the United States and some European countries that Iran has pursued nuclear weapons and could do so again in the future. Iran insists that it has answered all of the IAEA's questions.
Iran has rebuffed demands by the United Nations to suspend uranium enrichment, a process used for civilian energy but could eventually lead to enriching uranium to concentrations needed for weapons.
The IAEA report said Iran has not satisfactorily explained evidence that Tehran had developed missile technology for carrying nuclear warheads and pursued information on how to shape uranium for a bomb core and trigger a nuclear detonation.
The five permanent members of the Security Council - China, Britain, France, Russia and the United States - plus Germany have been leading the international effort to ensure Iran does not develop a nuclear weapons capability.
The Security Council has enacted limited sanctions on Iran over its refusal to suspend uranium enrichment, but they have done little to stop Iran from expanding its effort.
The head of EU foreign policy, Javier Solana, is due to present Iran with an offer from the six countries that includes incentives for Iran to comply, while also laying out consequences of an Iranian refusal.
Iran has so far been able to enrich uranium enrichment at a concentration of four per cent, well below the 90 per cent needed for weapons grade material.