FILE - Mohamed Toumba, one of the soldiers who ousted Nigerian President Mohamed Bazoum, addresses supporters of Niger s ruling junta in Niamey, Niger, Sunday, Aug. 6, 2023. AP
An army detachment was "the victim of a terrorist ambush near the town of Koutougou" in the Tillaberi region near Burkina Faso on Tuesday, said a defence ministry statement.
In the heaviest losses since the July 26 coup, another 20 soldiers had been wounded, six seriously, with all the casualties evacuated to the capital Niamey.
More than 100 assailants riding motorbikes, were "neutralised" during their retreat, the army said.
A jihadist insurgency has plagued Africa's Sahel region for more than a decade, breaking out in northern Mali in 2012 before spreading to neighbouring Niger and Burkina Faso in 2015.
The so-called "three borders" area between the three countries is regularly the scene of attacks by rebels affiliated with the Islamic State group and Al-Qaeda.
The unrest across the region has killed thousands of troops, police officers and civilians and forced millions to flee their homes.
Anger at the bloodshed has fuelled military coups in all three countries since 2020, with Niger the latest to fall when its elected president, Mohamed Bazoum, was ousted on July 26.
Alarmed by the cascade of takeovers, the West African bloc ECOWAS has warned of possible military intervention to reinstall Bazoum, who is being detained in the presidential compound in Niamey.
Military chiefs of the Economic Community of West African States (ECOWAS) are to meet in Ghana on Thursday and Friday to follow through a decision by their leaders last week to deploy a "standby force to restore constitutional order" in Niger.
Analysts say an intervention would be militarily and politically risky, and the bloc has declared that it prefers a diplomatic outcome.
ECOWAS issued a statement Tuesday "strongly condemning" the latest attacks.
It urged the military "to restore constitutional order in Niger to be able to focus (its) attention on security ... weaker since the attempted coup d'Etat."
The generals who have detained Bazoum said "the deteriorating security situation" sparked the coup.
Talks have taken place this week in Addis Ababa, gathering ECOWAS and Niger representatives under the aegis of the African Union.
On Tuesday, Niger's military-appointed civilian prime minister, Ali Mahaman Lamine Zeine, made an unannounced visit to neighbouring Chad, a key nation in the unstable Sahel but not a member of ECOWAS.
He met President Mahamat Idriss Deby Itno, handing over what he described as a message of "good neighbourliness and good fraternity" from the head of Niger's regime.
"We are in a process of transition, we discussed the ins and outs and reiterated our availability to remain open and talk with all parties, but insist on our country's independence," Zeine said.
He left for Niamey on Wednesday afternoon, Chadian authorities said.
Bazoum's election in 2021 was a landmark in Niger's history, ushering in the country's first peaceful transfer of power since independence from France in 1960.
He survived two attempted coups before being toppled in the country's fifth military takeover.
ECOWAS has applied a raft of trade and financial sanctions while France, Germany and the United States have suspended their aid programmes.
The measures are being applied to one of the poorest countries in the world, which regularly ranks bottom of the UN's Human Development Index, a benchmark of prosperity.
Niger is also facing a jihadist insurgency in its southeast from militants crossing from Nigeria, the cradle of a campaign initiated by Boko Haram in 2010.