"The Iranians are proving to the world that they have no desire to live up to their own responsibilities. That's alarming," said White House spokesman Robert Gibbs.
After months trying to engage Iranian leaders and persuade them to stop enriching uranium, US President Barack Obama's administration is now seeking fresh UN sanctions against the regime.
If Iran is unwilling to answer its responsibilities, "we will take the next set of steps," Gibbs warned Tuesday.
World powers suspect Iran is enriching uranium to make nuclear weapons under cover of its civilian energy program, a charge Tehran denies.
Enriched uranium can serve as fuel to power nuclear reactors or in highly refined form to produce the fissile core of an atom bomb.
Iran has so far snubbed a deal brokered last year by the International Atomic Energy Agency that envisages France and Russia supplying nuclear fuel for a Tehran research reactor if it hands over the bulk of its low-enriched uranium.
The head of the UN atomic watchdog, Yukiya Amano, said Monday that Iran was still not giving sufficient information on its nuclear activities but that the uranium fuel deal was still on the table.
Iran countered on Tuesday by accusing Amano of taking sides and called on the Japanese official to modify his stance.
Last month Amano circulated a blunt report to IAEA member states on Iran's atomic program in which he expressed concern Tehran that may already be working on a nuclear warhead.
He also confirmed Iran had started enriching uranium to higher levels, theoretically bringing it close to levels needed for an atomic bomb.
US Secretary of State Hillary Clinton said Monday it could still take months to agree a fourth round of UN sanctions against Iran, as key Security Council veto-wielding powers China and Russia still appear divided on the matter.
