"Obviously, they're ... heavily involved in trying to develop nuclear weapons enrichment, the enrichment of uranium to weapons grade levels," Cheney said in an interview with ABC television transcribed by the White House.

Cheney, however, did not mention on what he based his accusation.

The United States and its European allies have led efforts to pressure Iran into freezing its disputed uranium enrichment work, a process that can be used both to make nuclear fuel and the core of an atomic bomb.

Tehran insists its program is peaceful.

The UN Security Council recently imposed a third set of sanctions against Iran over its refusal to halt its nuclear activities.

Washington has stepped up pressure to halt Tehran's uranium enrichment program ever since a US intelligence report in December said Iran did have, in effect, a covert nuclear weapons program but that it was stopped in 2003.

The report, which the White House interpreted as confirming its suspicions about Iran's secret ambition, increased skepticism over Washington's warnings that began after the Iraq war did not yield the weapons of mass destruction the US had predicted.

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