Amadou Haya Sanogo in court along with 17 co-defendants for arbitrarily executing dozens of soldiers.
A dozen voice recorders handed out in Timbuktu in Mali as Islamist groups continue to operate.
Situation at the maximum security centre is unclear but Niger has often been attacked by members of the terror groups.
The kidnappers were said to be taking the hostage across the border to Mali.
Mahdi claimed he had been caught in an "evil wave" by al-Qaeda when he joined the Islamist group.
Ahmad al-Faqi al-Mahd first to face war crimes charges of its kind after 14 historic sites attacked.
Ansaru, a Boko Haram breakaway group is known for kidnapping and killing Westerners.
Islamist militia defending ethnic Peul group has also claimed responsibility for the attack.
Aqim demands the release of imprisoned fighters in Mali and Niger in exchange for Stockly's release.
Five UN peacekeepers were killed and 1 seriously injured in a terror attack on the day that is observed to honour them.
Since the signing of a peace accord in 2015, 12 UN peacekeepers have been killed in concerted attacks.
Country has become a hub for weapons and munitions smuggling across the region since 2011.
Latest deaths bring French army casualties to 17 since operations began in 2013.
Four gunmen attacked a hotel hosting an EU military training mission in Bamako, Mali's capital.
Rights groups claim Mali forces detaining thousands of conflict-linked civilians 'without legal warrant'.
Militants detonated the car bomb at around 6:30 outside the former hotel.
Al-Qaeda gunmen filmed storming the Splendid Hotel, burning cars, and firing in the air.
Migrants from Mali and Gambia walk down fashion runway at Florence's Pitti Uomo exhibition.
Pro-government Movement of Azawad group says four of its fighters killed in friendly fire.
Four or five rockets landed inside the base in the northern town of Kidal, said UN spokesman.
France says special forces sergeant injured during anti-jihadist operations in Mali has died.
Mourabitoun named the pair through the Mauritanian state news agency al-Akhbar.