Iranian Foreign Minister Mohammad Javad Zarif said on Tuesday that Tehran's negotiators were being assisted by experts in international law during the "difficult" nuclear talks being held in Vienna.

The Vienna meeting, which began on Tuesday, is aimed at reaching a lasting accord to end a decade-long standoff over Iran's nuclear programme.

"In the next round, we will start writing the text of a final agreement which will be a complicated, difficult and slow task," Zarif, who leads the Iranian negotiating team, wrote on his Facebook page.

"Therefore, I formed an advisory group in legal affairs which comprises many prominent and well-known professors specialised in international law ... to assist the negotiating team in that important matter," he said.

Western nations and Israel have long suspected Iran is pursuing a nuclear weapons capability alongside its civilian programme, charges denied by Tehran.

Iran and the five UN Security Council permanent members plus Germany want to transform a deal struck in November into a permanent agreement by the time this temporary accord lapses on July 20.

Under the November deal, which took effect on January 20, Iran froze certain nuclear activities for six months in exchange for minor relief from sanctions hurting its economy.

As part of the deal, Iran was given greater access to civilian aircraft parts and on Monday Washington said that Boeing had been issued with a temporary licence to do business with the Islamic republic.

Now the powers want Iran to reduce permanently, or at least long-term, the scope of its programme in order to make any dash to make the bomb extremely difficult and easily detectable.

Zarif had on Monday expressed hope to round up the negotiating phase with world powers during this week's talks and move on to drafting a final accord on the nuclear programme.