US Secretary of State John Kerry, still on crutches after breaking his leg, headed to Vienna Friday looking to seal a legacy-making nuclear deal with Iran.

The indefatigable 71-year-old boarded his Boeing 757 for the first time since he fell off his bike in late May and broke his right femur while cycling in the French Alps. That required an operation and kept Kerry in the hospital for almost two weeks.

Kerry left for the last leg of marathon talks aimed at hammering out an unprecedented accord with Iran to curb its nuclear program, and put a bomb out of its reach.

Iran and six global powers leading the negotiations -- Britain, China, France, Germany, Russia and the US -- have given themselves until Wednesday to reach a final comprehensive accord.

Under such an agreement Iran would scale back its nuclear activities in return for an easing of sanctions.

A senior US official admitted for the first time Thursday however, that the talks could slip beyond the June 30 deadline, insisting that it was important to get the right deal.

"We always knew that as we got towards the end it would get tougher and tougher because the stakes get higher," the official told reporters travelling with Kerry.

"You always leave the most difficult issues to the last, and as you all know nothing is agreed until everything is agreed."

The eyes of the world will be on Vienna in the coming days to see if a nuclear deal with Iran can be struck. That would end a 12-year standoff with the West and signal the possibility of a new place on the global stage for the Islamic republic.

It would also be a coup for Kerry, and the months of diplomacy he and his team have invested in knitting together what could prove to be one of the world's most significant non-proliferation treaties.

Talks are due to begin Saturday with Iranian Foreign Minister Mohammad Javad Zarif, with other members from the so-called P5+1 group due to arrive over the weekend.

Kerry boarded the plane Friday on crutches and riding in a special enclosed hydraulic compartment which lifted him up to the aircraft. He is being accompanied to Vienna by his doctor and a physical therapist.