Four Frenchmen kidnapped by Al-Qaeda-linked militants in Niger have been released after more than three years in captivity.

The exact circumstances of their release were not immediately clear, but French Defence Minister Jean-Yves Le Drian said there had been "no assault" to free the hostages and that no ransom had been paid.

Foreign Minister Laurent Fabius told AFP the hostages had been freed in Mali and were in "very good shape".

"They have been hostages for three years and the nightmare is finally over," Fabius said.

The four men, who were kidnapped by Al-Qaeda in the Islamic Maghreb (AQIM) in northern Niger in 2010, arrived at the airport in the capital Niamey on Tuesday, where they were greeted by the French foreign and defence ministers and by Niger's President Mahamadou Issoufou.

They appeared thin but otherwise in good health, an AFP reporter at the scene said.

In a brief remark to AFP at the airport one of the hostages, Thierry Dol, 32, said: "It was very difficult but it was the test of a lifetime."

Francoise Larribe, who was kidnapped along with her husband Daniel before she was freed in February 2011, said: "It's an emotional wave, a tsunami", on learning of her husband's release.

"I have never lost hope, even though there were moments of dejection, fear and anguish," she said.

French President Francois Hollande had hours earlier announced their release during a visit to Slovakia's capital Bratislava.

"I have some good news. I just learned from Niger's president that our four hostages in the Sahel, the Arlit hostages, have been released," Hollande said.

Hollande is due to meet them on their arrival at an airport in a Parisian suburb on Wednesday.

Frenchmen Dol, Larribe, Pierre Legrand and Marc Feret were kidnapped on September 16, 2010, from a uranium compound in Arlit, north-central Niger.

Hollande spoke of "three years of trials for the kidnapped men, who were held by unscrupulous captors", and of "three years of suffering for the families who lived through a nightmare and are now relieved."

"I want to express my gratitude to Niger's president, who was able to obtain the release of our countrymen."

Speaking as he met the ex-hostages, Issoufou said Niger had worked for their release, but provided few details.

"Since the kidnapping of the hostages three years ago, Niger has worked on obtaining their release. Now it's done," he said, congratulating the hostages for "regaining their freedom after months of difficult trials."

Legrand's mother, Pascale Robert, told BFMTV that "it's like feeling something that we've never felt".

"Now we're waiting for them to physically return, to see them, to touch them."

'Negotiations in Mali desert'

The news of their release came days after regional security sources in the town of Gao in neighbouring Mali reported the presence of envoys in the Sahel "to speed up negotiations towards freeing the French hostages".

France had officially denied sending envoys.

According to a high-ranking Nigerien source, the four were taken to Niamey by a French plane from Anefis, in northeastern Mali near the Algerian border.

That was the site of final negotiations, which included Mohamed Akotey, the Nigerien management board chairman of Areva subsidiary Imouraren SA.

The hostages were apparently held in different locations for fear that they could all be freed during a single French offensive. They were brought together just days before the release.

A Malian security source also said that "the final negotiations took place in the Malian desert," adding that "eminent Malians in the north provided timely assistance".

Bamako welcomed the release but made no mention of any negotiations or whether it took part.

"Malian authorities congratulate ... the Nigerien president who worked tirelessly to obtain the liberation" of the four, it said in a statement read out on Malian public television.

The European Union's foreign policy chief Catherine Ashton also thanked Nigerien authorities for their "crucial contribution".

Three other people who were kidnapped at the time -- Francoise Larribe, a Togolese and a Madagascan -- were freed in February 2011.

AQIM had demanded at least 90 million euros ($124 million) for the release of the remaining hostages.

At least seven French hostages remain in captivity around the world, including two snatched in Mali, one in Nigeria and four in Syria.

AQIM released a video in September purporting to show seven kidnapped Westerners, including the four Frenchmen, in footage that France's foreign ministry deemed credible.

The video included statements from the four, as well as from a Dutchman, a Swede and a South African who were abducted from Timbuktu in northern Mali in November 2011.

The fates of the other foreign hostages were not clear.

AQIM grew out of a movement launched in the late 1990s by radical Algerian Islamists who sought the overthrow of the Algerian government to be replaced with Islamic rule.

The organisation linked to Al-Qaeda in 2006 and has spun a tight network across tribes, clans, family and business lines that stretches across the vast Sahel region abutting the southern Sahara desert.

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