Seven soldiers were killed in combat against hundreds of insurgents on motorbikes while five others died in an accident while driving to reinforce the targeted unit, the statement said.
The attack took place about 190 km (118 miles) from the capital Niamey in Kandadji, near the tri-border zone of Mali, Burkina Faso and Niger, the epicenter of Islamist insurgencies in the Sahel region over the last few years.
Earlier on Thursday, three sources including a senior military officer, who requested anonymity because they are not authorized to speak to the media, told Reuters that at least 10 soldiers were killed.
Neither the sources nor the defense ministry said which group was responsible. Local affiliates of al-Qaida and Islamic State are active in the region and wage frequent attacks on soldiers and civilians.
The defense ministry statement said around a hundred insurgents were killed and their motorbikes and arms were destroyed. It gave no further details.
Two security sources said the army responded to the attack with ground troops as well as helicopters, one of which was hit but was able to return to its base.
Niger is run by a military junta that seized power in a coup in July, partly out of discontent at the worsening security situation. Neighboring Mali and Burkina Faso have each had two coups in the last three years.
However, security analysts say attacks had been falling in Niger under ousted President Mohamed Bazoum, who had tried to engage with Islamists and the rural communities where they are rooted.
At least 17 soldiers were killed in another attack in southwestern Niger in mid-August.
FILE - Supporters of Niger's junta rally near the French military base calling for the withdrawal of French forces, in Niamey, Niger, Sept. 28, 2023.
France said on Sunday it would withdraw its 1,500 troops from Niger before the end of the year, after weeks of pressure from the junta and popular demonstrations against the former colonial ruler, which had forces there to fight the insurgents.
On Thursday, several hundred pro-junta supporters gathered again in front of the French military base in Niamey to demand that the troops leave.