The United States voiced renewed concern Thursday about Iran's nuclear program after a report by the UN's atomic watchdog suggested Tehran may be working on a nuclear warhead.

US President Barack Obama's spokesman said the latest reports by the International Atomic Energy Agency (IAEA) continues "to demonstrate the failure of the Iranian government to live up to its international obligations."

"The president has on a number of occasions talked about engagement, talked about the benefits of living up to those international obligations," White House spokesman Robert Gibbs told reporters aboard Air Force One.

"We always said that if Iran failed to live up to those international obligations that there would be consequences."

The Vienna-based IAEA said earlier Thursday in a restricted report obtained by AFP that Tehran may be working on a nuclear warhead and had begun enriching uranium at higher levels.

"We have ongoing concerns about Iran's activities," State Department spokesman Philip Crowley told reporters.

"We cannot explain why it refuses to come to the table and engage constructively to answer the questions that have been raised," he said.

The 10-page document, which is to be discussed by IAEA governors at a meeting next month, also confirmed Tehran had begun enriching uranium to higher levels, theoretically bringing it closer to the levels needed for an atomic bomb.

But a senior US official said that the report also showed that Iran was enriching uranium at a level which would take it "years" to produce a nuclear weapon.

The official, speaking on condition of anonymity also added that the IAEA assessment demonstrated the "significant" technical problems that were hampering the Iranian nuclear program.

He also added that the report showed an "increasing pattern of non-cooperation with the IAEA" on the part of Iran.

Iran has previously reached uranium enrichment levels of no more than five percent at its facility at Natanz, in defiance of UN orders for it to cease and despite three rounds of UN sanctions.

Earlier this month, Iran announced it would begin enriching uranium to 20 percent, ostensibly to make the fuel for a research reactor that makes medical radioisotopes.

On Wednesday the top US military officer, Admiral Mike Mullen, warned that Iran was "becoming a nuclear weapons capable country and that is very dangerous," although he stressed that the Obama administration's priority was to initiate dialogue and engagement with Iran.

The report was the first assessment on Iran under the oversight of new IAEA Director General Yukiya Amano.

Amano sees his job as "just the facts," the official said, adding that he had a more technical approach than former IAEA director general Mohamed El Baradei.