NIAMEY, Niger — Niger’s president has fired the army’s chief of staff after attacks against security forces have killed at least 174 security force members since December.

President Issoufou Mahamadou’s action on Monday came after the death toll from an attack by Islamic extremists on Niger’s military last week rose to at least 89, making it the most deadly attack of its kind in years in the West African nation.

The fatalities from the attack Thursday rose dramatically from the 25 soldiers that the government initially said were killed last week.

In addition to the soldiers killed in that attack on the Chinagrodrar Advanced Military post near the border with Mali, at least 77 extremists were killed by Niger’s army and its foreign allies, most notably France and the United States, according to a statement issued by the government.

Three days of national mourning began Monday, the government said.

Chief of Staff Lt. Gen. Ahmed Mohamed will be replaced by Brig-Gen. Salifou Modi, who was the military attache for Niger in Germany, a presidential statement said Monday. Mahamadou also dismissed the Secretary General of the Ministry of National Defense and the Chief of Land Staff.

No group has yet claimed responsibility for Thursday’s attack, but it bore the hallmarks of an Islamic State-linked group that said it was behind the December ambush near the town of Inates that killed 71 soldiers and was previously the most deadly attack of its kind in Niger in years.

“The government calls on the population to be more vigilant, more serene and united, and reaffirms its determination to continue the fight against terrorism until the final victory,” the government statement said.

Dalatou Mamane is an Associated Press writer.