NIAMEY U.N. Secretary-General Ban Ki-moon urged Niger's government to reinforce security around refugee camps after gunmen killed 22 soldiers stationed at a camp for 4,000 Malian refugees.
Thursday's attackers also burned an ambulance and looted a health centre at the Tazalit camp in Niger's western Tahoua region, bordering Mali, a U.N. statement said on Friday.
The Malian refugees were unharmed, the United Nations refugee agency UNHCR said.
The camp shelters Malians who have fled to Niger since Islamist militants, some with links to al Qaeda, seized Mali's desert north in 2012.
A French-led military intervention pushed back the insurgents a year later but clashes with rebels and Islamist attacks have led more people to flee. More than 60,000 Malian refugees now live in Niger.
Last month, a Malian woman and child were killed when unidentified gunmen attacked a security post at another refugee camp in Niger, the United Nations said.
Niger's government said it had launched an investigation into the Tazalit attack and declared two days of national mourning.
"The president of the republic (and) the government present, in the name of the devastated Nigerien people, their condolences to the families of the victims," said a statement read on national radio.
Niger's small army is not only seeking to prevent armed groups and bandits entering from Mali to the west, but also fighting Boko Haram militants launching raids from Nigeria in the south.
There are also concerns that Islamic State fighters could be pushed into Niger by a government offensive in Libya to the northeast.
(Reporting by Boureima Balima; Writing by Nellie Peyton; Editing by Aaron Ross and Dominic Evans)