"Iran's nuclear weapons programme and the now well-documented pattern of its (Non-Proliferation Treaty) safeguards violations are deeply troubling," spokesman Steven Pike said.
The International Atomic Energy Agency (IAEA), headed by Mohamed ElBaradei, has accused Iran of covert nuclear activities over the past 20 years, including making plutonium, but said there is as yet no evidence it is trying to build an atomic bomb.
"Dr ElBaradei's report reinforces our concerns. Iran's defiance of its NPT safeguards obligations present a serious challenge to the IAEA and to the NPT-based nuclear non proliferation regime," Pike said.
The confidential IAEA report was released Monday ahead of a November 20 meeting of the body's board of governors, which is set to rule on Iran's nuclear program after a months-long standoff.
The United States accuses its arch-foe of secretly trying to develop atomic weapons under the cover of a nuclear power program.
It wants the IAEA's 35-nation board to judge Iran in non-compliance with its treaty obligations and send the matter to the UN Security Council, which could impose sanctions.
"Iran has no peaceful need for uranium enrichment and plutonium reprocessing and cannot rebuild international trust if it continues to pursue them," Pike said.
"It is important that Iran suspend all uranium enrichment-related and reprocessing activities in the country," he said.
"Until Iran has fully satisfied the IAEA questions, and fully addressed the international community's concerns, we believe no country should be engaging in nuclear cooperation with Iran," he added.
Iran, which insists its nuclear activities are for civilian purposes only, acknowledged on Tuesday that its nuclear program had breached safeguard rules from the nuclear Non-Proliferation Treaty, but asserted the failures were only minor and a thing of the past.