The United States warned Iran on Thursday its window for opening serious talks is limited, after the UN nuclear watchdog said Iran had doubled enrichment capacity at its underground facility at Fordo.
White House spokesman Jay Carney also reiterated that President Barack Obama is determined to prevent Iran getting a nuclear bomb but added that US officials would know whether it reached breakout capacity to reach that stage.
"The window of opportunity to resolve this remains open ... but it will not remain open indefinitely," Carney said.
The International Atomic Energy Agency said in its latest quarterly report, seen by AFP but not published, that Iran now has now installed at Fordo around 2,000 uranium-enrichment centrifuges, compared with around 1,000 in May.
It also said its ability to inspect the Parchin military base where it suspects nuclear weapons research took place in the past had been "significantly hampered" by a suspected clean-up.
The report was published at a point when talks between world powers and Iran on its nuclear program are effectively stalled and with rising speculation that Israel may launch a unilateral military strike on Iranian facilities.