World powers were set Tuesday to miss yet another deadline to nail down an elusive nuclear deal ending a 13-year standoff with Iran, despite hours of difficult top-level negotiations.

In a sign of how complex the negotiations have become, foreign ministers met deep into the night Monday grappling with the toughest remaining issues which have so far thwarted a deal to curtail Iran's nuclear programme.

Despite progress on a series of complicated annexes, the negotiations have bogged down on how to ease a web of sanctions against Iran, probing allegations that in the past Tehran sought to develop nuclear arms, and ensuring Iran can continue to have a modest, peaceful nuclear program.

There was hope that a long-stalled probe into allegations that Iran, before 2003 and possibly after, sought to develop nuclear arms could finally be concluded after Yukiya Amano, the head of the International Atomic Energy Agency (IAEA) travelled to Iran last week.

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