US Secretary of State John Kerry has a history with broken planes but during Iran nuclear talks in Switzerland this week even his bike needed fixing.

In what sceptics might call a metaphor for the talks in Lausanne, Kerry's orange racer on Monday couldn't get into gear properly, Lionel Schumann of the Sam's Bike shop in the town of La Tour-le-Peilz said.

"The Swiss police came in and told me they needed an urgent repair on a gear-change mechanism," Schumann told AFP on Wednesday.

"Swiss and US security people came in and inspected the shop -- the entrances, the exits. Then Mr Kerry arrived. Happily for me he speaks very good French and was able to explain the problem," he said.

The bike, he said, was a good model even though it was around 10 years old, Schumann told Le Matin newspaper which first reported the story.

This is around the same age as the standoff with Iran over its nuclear drive that Kerry is trying to bring to a close with his Iranian counterpart Mohammad Javad Zarif.

Iran and six powers -- the US, Russia, China, Britain, France and Germany -- are seeking to agree by March 31 the outlines of a deal that they hope will bury fears that Iran might develop nuclear weapons.

On Tuesday White House spokesman said the chances of such a deal are 50/50 "at best" with "some of the most difficult issues... yet to be resolved."

Kerry's bike problem though was quickly fixed and although Schumann refused to take any money, the fit 71-year-old American insisted. "They gave us $20," Schumann said.