VIENNA • After almost two years of talks, there seemed no hope of meeting a nominal deadline of midnight, June 30, for an accord with world powers on Iran's nuclear programme.

But diplomats remain optimistic that an accord is within reach and will need negotiations that will take only a few more days.

Iran's Foreign Minister Mohammad Javad Zarif was back in Vienna yesterday for the nuclear talks, after a day of consultations in Teheran, Iranian officials said.

And he brought a high-powered team including Iran's nuclear chief Ali Akbar Salehi, who has been absent after undergoing intestinal surgery last month.

Also in the group is President Hassan Rouhani's brother and confidant Hossein Fereydoun.

"This is the most crucial round of this 22-month long process," said Mr Ali Vaez, senior Iran analyst at the International Crisis Group. "It is not surprising that the Iranians need to have all hands on deck."

Mr Zarif was due to meet US Secretary of State John Kerry yesterday, officials said.

Russian Foreign Minister Sergei Lavrov was also due in Vienna later in the day, while other foreign ministers were expected in the coming days.

Iran and the P5+1 powers - Britain, China, France, Germany, Russia and the United States - agreed a framework accord in April that they are now seeking to finalise.

The deal will see Iran scale down its nuclear programme in order to make it virtually impossible to build a nuclear bomb. In return, painful sanctions will be lifted.

Tough issues remain, however, including working out the pace and timing of sanctions relief and increased access in Iran for the United Nations atomic watchdog to carry out inspections, as well as the nature of such checks.

AGENCE FRANCE-PRESSE, BLOOMBERG