Niger's government has said that about 22 inmates, including several "terrorists", escaped a prison in the capital Niamey after armed men attacked the jail.
The statement came on Sunday, a day after officials said the escape bid had failed.
Marou Amadou, the justice minister, who also acts as government spokesman, said that inmates facing terrorism charges had killed three prison guards and seriously wounded three others during the shootout on Saturday.
"It has emerged from initial investigations at the site that the aggressors obviously benefitted from outside complicity regarding the weapon introduced into the prison," Amadou said.
Among the escapees was Cheibane Ould Hama, who was imprisoned for killing four Saudis and a US citizen, Amadou said, adding that Hama was "actively sought".
Hama was sentenced to 20 years in prison in June 2012 for his involvement in an attack on a group of tourists on a hunting safari that killed four Saudi nationals in December 2009 and the killing of an American in a bar in Niamey in 2000.
The escaped prisoners are "a danger for the region", a Malian security source said.
The prison attack 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 armed groups - Movement for Oneness and Jihad in West Africa (MUJAO) and Signatories in Blood - claimed responsibility for the attacks, saying they were in retaliation for Niger sending troops to participate in a French-led military operation against al-Qaeda-linked fighters in neighbouring Mali.
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 on Sunday when they opened fire on what they said was a suspicious-looking 4WD vehicle 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..
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.