Four Frenchmen held hostage in the Sahara desert by al Qaeda-linked gunmen for three years left Niger on a French government plane this morning.

The men, who were kidnapped in 2010 while working for French nuclear group Areva and a subsidiary of construction group Vinci in northern Niger, were freed on Tuesday after secret talks.

A Reuters correspondent at Niamey airport said the four men boarded the jet with two French ministers, including Foreign Minister Laurent Fabius, dispatched to pick them up.

"I am very happy. It was difficult, the ordeal of a lifetime," said Thierry Dol, one of the freed men.

Fabius said the men were in a state of shock, having been isolated for so long. "They slept well, but on the floor as they are not yet able to sleep on mattresses," he said.

The men's release gave Francois Hollande a boost a day after a poll showed he had become the most unpopular French president on record.

No details have been given on the circumstances of the quartet's release but Niger's President Mohamadou Issoufou said they had been retrieved from northern Mali.

Thousands of French troops were dispatched to Mali's desert north earlier this year to prevent Islamists and criminal gangs operating in the zone who occupied the region in 2012 from extending their reach further south.