They included Cheibane Ould Hama of Mali, who was convicted of killing four Saudis and a US citizen, Justice Minister Marou Amadou told a press briefing, adding that Hama was “actively sought”.
Amadou, who also acts as government spokesman, confirmed that inmates facing terrorism charges had killed three guards during the shootout on Saturday.
The breakout happened a little over a week after twin suicide bombings on an army base and French-run uranium mine claimed more than 20 lives in northern Niger on May 23.
Two jihadist groups— Movement for Oneness and Jihad in West Africa (MUJAO) and Signatories in Blood— claimed the attacks, saying they were in retaliation for Niger sending troops to participate in a French-led military operation against Al-Qaeda-linked extremists in neighbouring Mali.
The escaped prisoners are “a danger for the region”, a Malian security source said.
“It has emerged from initial investigations at the site that the aggressors obviously benefited from outside complicity regarding the weapon introduced into the prison,” said Amadou, the Niger justice minister.
He called on the west African country’s people to “remain calm” and exercise their “duty to be vigilant”.
In a separate incident, officers from Niger’s anti-terror squad killed one person and wounded another Sunday when they opened fire on what they said was a suspicious-looking four-by-four with tinted windows that had been driving back and forth in front of their headquarters.
Amadou said officers had given the “usual warnings” before firing shots in order to stop the vehicle.
The car’s other two occupants have been taken into custody, he said.
Residents said the vehicle may have simply been cruising around central Niamey in what local youths call a “rodeo”.
Cheibane Ould Hama, considered the most-wanted among the escapees, was convicted of killing four Saudi tourists in an attack on a convoy travelling near the border between Mali and Niger in 2009.
He was also convicted of killing an American national in 2000 in front of a bar in Niger’s capital.
Niger has seen repeated kidnappings and attacks on its territory by Al-Qaeda in the Islamic Maghreb (AQIM) in recent years.
Signatories in Blood, one of the groups that claimed the May 23 bombings, has threatened to carry out further attacks on Niger and other countries with troops in Mali.
Islamist extremists had seized control of Mali’s vast desert north for 10 months before the French-led intervention launched in January drove them out.