Talks resumed on Thursday between Iran and world powers about a nuclear programme that the West suspects is aimed at nuclear bomb research.

A first day of discussions showed a "fair amount of disagreement" but also areas of common ground, a senior U.S. official said on Wednesday, adding: "I believe we have the beginning of a negotiation."

Talks got under way again on Thursday morning, a diplomat said.

Both sides have been publicly upbeat about the scope for an outline deal following a 15-month diplomatic freeze and exploratory talks in Istanbul last month.

But in Vienna, western diplomatic sources said a U.N. watchdog report was expected to show Iran has installed more uranium enrichment centrifuges at an underground site, potentially boosting output capacity of the nuclear work the major powers want it to stop.