A journalist and rights activist known for his outspoken criticism of the humanitarian crisis in south-eastern Niger has been arrested for "collaborating" with Boko Haram Islamists, the interior minister said on Wednesday.

Human rights watchdog Amnesty International condemned Moussa Tchangari's arrest and called on Niger to release him.

"This man has been collaborating with Boko Haram for some time, and he is actively spreading propaganda and false news in liaison with Boko Haram," Interior Minister Hassoumi Massaoudou told AFP.

"All his propaganda aims to show... that Niger's defence and security forces are the criminals... [and] not Boko Haram."

Niger cannot "tolerate such an active collaboration with terrorists", or such "systematic spreading of false news", he added.

Tchangari was arrested on Monday and charged with "criminal links to the terrorist group Boko Haram", he said.

Tchangari's organisation Alternative Espace Citoyen has been critical of the humanitarian crisis in south-eastern Niger, where the army is fighting Boko Haram.

In early May, his group published a report that criticised the Niger authorities after the evacuation of some 25 000 Lake Chad residents over fears of new Islamist attacks, following a deadly assault in late April.

At the time, Tchangari said thousands of men, women, children and elderly Lake Chad residents "walked for more than 50km" until they reached safety.

"No preparations were in place to welcome... or support them," he added.

In early May, a UN source said the evacuees were living in "dramatic" conditions - without tents or shelter, and in some cases without access to drinking water.

In a statement on Tuesday, Amnesty International called on Niger to free Tchangari "immediately", saying: "The fight against Boko Haram must not serve as a pretext to violate free speech."

The call for Lake Chad residents to evacuate came a week after a cross-border assault by Nigerian-based Boko Haram insurgents on the island of Karamga that left at least 74 people dead.

It was Niger's heaviest loss since it joined a regional offensive against the militants, whose six-year insurgency has claimed some 13 000 lives and displaced about 1.5 million people