Top diplomat John Kerry is committed to reaching a nuclear deal with Iran and plans to join the final negotiations in late June despite breaking his leg in a serious cycling accident, US officials said Monday.

As Kerry headed back to Boston from Geneva in a military C-17 plane accompanied by his personal doctor, observers said the timing of his fall came at a bad time as America juggles multiple foreign policy challenges, but was unlikely to derail the Iran talks.

The lanky 71-year-old, who is an experienced cyclist, broke his right femur on Sunday when his bike hit a curb as he started on a climb of a tricky, steep Alpine mountain pass near the French town of Chamonix.

His deputy spokeswoman, Marie Harf, said Kerry was in "good spirits" Monday and had spoken to several European counterparts to apologize for cancelling stops in Madrid and Paris.

"He's committed to an aggressive, ambitious, and responsible recovery timeline," she told reporters.

"Look fwd to getting leg set & getting back to @StateDept! Meantime, work goes on. Big thanks for well-wishes. #Onward," Kerry said in a message on his Twitter account.

Another State Department official, who asked not to be named, confirmed Kerry will have to undergo surgery.

But doctors told AFP that in such cases a patient could be up and walking within three to four days, and would be expected to make a full recovery in two months.

Already Kerry is planning to "remotely" join a key conference on combating militants from the Islamic State (IS) group which he had been scheduled to attend on Tuesday in Paris.

- Focus on Iran -

But all eyes are on the looming June deadline to reach a deal curtailing Iran's suspect nuclear program and end a 12-year standoff with the Islamic republic.

Even though the talks have involved a large American team, Kerry has over the past 18 months personally invested time and energy in the highly complicated negotiations, which could prove a lasting legacy of his tenure as secretary of state.

He has met many times with his Iranian counterpart Mohammad Javad Zarif, and the accident happened the day after they spent six hours locked in "intense" talks in a Geneva hotel.

Even though their countries do not have diplomatic relations, the two men have got to know each other well, as they have tussled during tense all-night sessions chasing a deal.

"Secretary Kerry's main focus for the month of June remains squarely on the Iran negotiations. His injury does not change that" Harf told reporters, adding he had already spoken with Zarif since tumbling off his bike.

"He and the entire team are absolutely committed to the same timetable and are working toward June 30th as the deadline for these talks."

But she acknowledged the logistics for future upcoming talks had not yet been finalized.

"Personal relationships matter, but I think what has held the talks together all this time has been a recognition that it is in the interests of both countries to get the nuclear issue resolved," said Alireza Nader, international policy analyst with Rand corporation.

"The US team is pretty big and the US government has invested a lot of effort in this. I don't think the negotiations are necessarily dependant on one person."

Technical experts from the US, and Iran as well as Britain, China, France, Germany and Russia, have met almost continuously since an April 2 framework for a deal was laid down in Lausanne.

"Some of the issues that are now being discussed are probably some of the most difficult," Nader told AFP, highlighting there was still no agreement on lifting a network of sanctions against Iran or for inspecting its military sites.

- Recovery time -

Dr Sam Barzideh, director of the Orthopaedic Fragility Fracture Service at Winthrop-University Hospital in New York, warned that since Kerry had had a previous hip operation on the same leg his recovery could be complicated.

"A fracture needs to heal and that usually takes about two months, the bone union is about two months under the best of circumstances," Barzidah told AFP, cautioning he was not privy to Kerry's medical records.

But he said the aim was to get the patient "to weight-bear as early as possible. The whole idea is for him to be able to walk the next day or within two to three days."