US forces were attacked Wednesday near the Niger-Mali border.
"The body of another US service member has been recovered from the area of the attack, bringing the number of USA service members killed in this attack to four", Colonel Robert Manning said in a statement.
Wednesday's attack came after USA and Niger forces met with local tribal members about 200 kilometres (120 miles) north of Niger's capital, Niamey, near its border with Mali.
"The joint patrol was ambushed by terrorist elements in a dozen vehicles and on 20 motorbikes", Niger's Defense Ministry said in a statement.
US assistance to Niger's army, however, includes "intelligence, surveillance and reconnaissance in their efforts to target violent extremist organisations in the region", she said.
"Our country is once again the target of a terrorist attack, with a large number of victims", he said.
Now, they are knitted together as the first American troops to die in combat as part of the United States' broadening counterterrorism mission in Niger, which is in northwest Africa. For years, the USA army has offered training to the Niger's soldiers to enable them to fight the Islamist militants that have dominated the North Africa region including Mali, Niger, and Nigeria.
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About 800 US personnel are assigned to posts in Niger, mostly at two sites focused on gathering aerial reconnaissance for Nigerian forces.
The U.S. Africa Command offered few details on Wednesday's attack about 124 miles north of Niamey near the Mali border.
"It's a pretty broad mission with the government of Niger in order to increase their capability to stand alone and to prosecute violent extremists", the US military's Joint Staff Director, Lieutenant General Kenneth McKenzie, said on Thursday. Four Niger security forces were also killed. That effort has increased in recent years, said McKenzie.
The suspected militants who launched the raids belong to the al-Qaeda in Islamic Maghreb - a group affiliated to the al-Qaeda that is active in the North Africa region. "I don't have the exact numbers, but yeah, because Niger is an important partner of ours". The force gained U.N. Security Council backing in June but lacks funding.
US President Donald Trump has been briefed by his chief-of-staff, John Kelly, about the attack, White House spokeswoman Sarah Sanders said. The countries have been pressing the global community to help funding and in equipping troops and ensuring their mobility and help with logistics, communications and protection in the field.
In Niger, Washington has deployed around 800 soldiers, runs a drone base in the capital Niamey and is building a second in Agadez at a cost of around $100 million.