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

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

The ministers met twice on Monday with the Iranian delegation, led by Foreign Minister Mohammad Javad Zarif, for a total of almost three hours.