The United States sees Iran's attitude over the diplomatic crisis triggered by its suspect nuclear program as "alarming," the White House said on Tuesday.
"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.
"And if they are unwilling to do that, as we said, we will take the next set of steps."
After months spent trying to engage Iranian leaders in the hope that they would allay Western concerns about their suspect nuclear program, US President Barack Obama is now seeking tougher UN sanctions against the regime.
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 is at loggerheads with world powers for not accepting a deal that would supply it with nuclear fuel for a Tehran research reactor in return for the transfer of the bulk of its low-enriched uranium.