VIENNA - Iran is believed to have increased the number of centrifuges in an underground nuclear plant by nearly a third in three months, diplomats say, underlining the tough task facing major powers pressing Tehran to curb its atomic activity.

But despite a major expansion of its capacity to refine uranium at the Fordow facility, buried deep inside a mountain, Iran does not appear to have switched on its newly installed enrichment centrifuges yet, although this could happen soon, they said.

A UN report on Iran's nuclear program is also likely to show this week that it continues to use some of its most sensitive material for reactor fuel, at least temporarily making it unavailable for any quick atom bomb bid.

This could help in "buying more time for dialogue", a Western diplomat said, referring to international efforts to seek a diplomatic solution to the decade-old dispute and avert the threat of a new Middle East war.

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