Niger's government says attackers who killed at least 22 soldiers at a Malian refugee camp likely came from Mali.

The government said Friday it is investigating the attack a day earlier that wounded at least five at a security post for the Tazalit camp, in the Tahoua region of Niger near the border with Mali.

United Nations Secretary-General Ban Ki-moon on Friday urged Niger to reinforce security at refugee camps and other vulnerable targets. He also called on authorities to bring perpetrators to justice.

Northern and central Mali remain unstable nearly four years after France led a military intervention to drive out Islamic extremists.

Niger's government has declared Saturday a national day of mourning. The nation is also battling against Nigeria-based Boko Haram extremists in its south.