Niger courts released a journalist and human rights activist on bail after he was arrested for "collaborating" with Boko Haram Islamists earlier in the week, his lawyer told AFP.

Moussa Tchangari was arrested Monday when the country's Interior Minister Hassoumi Massaoudou announced he was being charged with "criminal links to the terrorist group Boko Haram".

Human rights watchdog Amnesty International earlier called on Niger to release Tchangari, whose organisation Alternative Espace Citoyen has been critical of the humanitarian crisis in the southeast of the country, where the army is fighting the Islamist group.

On Wednesday Tchangari was brought before a judge in the capital Niamey, his lawyer Me Nassirou Laouali said.

"The judge charged Moussa Tchangari, but released him on bail," Laouali said.

He added that prosecutors had "updated" the charges that were initially filed against the journalist, saying that he would now be charged with "violating national security".

Nouhou Arzika, another civilian who has been been in custody since Sunday, was also charged with violating national security and released on bail, state prosecutor Boukari Sally Ibrahim said.

Tchangari and Arzika had "made claims (in the media) which would demoralise the army during this period of war on terror," the prosecutor said.

Ibrahim added: "The judge will investigate the case. If he finds there are sufficient grounds, a trial will be brought."

In early May, Tchangari's organisation published a report that criticised Niger's 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 Lake Chad residents -- including women, children and elderly -- "walked for more than 50 kilometres (30 miles)" until they reached safety.

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

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

In a statement 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."