Some 22 prisoners, including Islamist militants, escaped from a prison in Niger's capital after several gunmen attacked the jail and killed two guards, the government said on Sunday.
The prison break comes a week after al Qaeda-linked groups raided a uranium mine and an army barracks in Niger, raising fears that the conflict in neighboring Mali could spread to other West African states.
Among the 22 prisoners unaccounted for were some convicted on terrorism charges including Alassane Ould Mohamed who was serving a 20-year sentence for the murder of four Saudi Arabians and an American, the government said in a statement.
"This individual is actively being sought," Niger's justice minister and government spokesman Marou Amadou said in the statement. "We are asking for the cooperation of the people to intercept them."
The government said two guards were killed during the raid on the prison on Saturday and three were seriously wounded, while two assailants were killed and two others arrested.
Mohamed was sentenced 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.
Officials had said on Saturday night the escape bid had failed.
In a separate incident on Sunday, security forces opened fire on a suspect vehicle which had driven several times around a building housing the country's anti-terror police unit.
One person in the vehicle was killed and another was wounded, the statement said, giving no further details.
Niger has emerged as a firm ally of France and the United States in the fight against al Qaeda-linked groups in the arid Sahel region.
Its army has deployed 650 troops in neighboring Mali to take part in a French-led war on armed Islamists groups who seized the northern two-thirds of the country last year.
The government has sought to shut its porous desert borders to Islamist groups that are thought to have shifted their bases to southern Libya and has allowed the U.S. to establish a drone base on its territory.