US Secretary of State John Kerry has left Switzerland for Boston, where he will be treated for a broken leg suffered in a cycling accident.

"Secretary Kerry has departed Geneva en route to Boston, Massachusetts," State Department spokesman John Kirby said on Monday.

Kerry, 71, a keen cyclist, had been hospitalised in Geneva - where he met his Iranian counterpart Mohammad Javad Zarif a day before Sunday's accident across the border in the French Alps.

He was initially expected to return to the United States on Sunday evening, but his flight was postponed, "for observation overnight for purely precautionary measures", the State Department said.

A yellow ambulance helicopter was seen flying from the Geneva hospital where he had received treatment to the nearby airport, where a US military C-17 aircraft stood waiting on the tarmac, an AFP photographer said.

Kirby said the plane was expected to arrive in Boston "later this evening".

As the plane took off, Kerry himself tweeted, in French, his gratitude to those who had helped him in Switzerland and France after his ordeal.

"I am particularly grateful to the police and remarkable medical teams in Geneva and Scionzier. True professionals," he said, referring to the French Alpine village where he fell off his bike Sunday morning.

Kerry's orthopaedic surgeon, Dennis Burke, who performed past hip surgery on him, as well as military medical personnel, were onboard with him.

The top US diplomat's fall happened after he had just set out on a planned ride of one of the stages of the Tour de France, the steep and winding Col de la Colombiere mountain pass, a local source said.