DAKAR (Reuters) - U.S. and Niger troops killed 11 Islamist militants from a Boko Haram splinter group in a firefight in December, the U.S. military said on Thursday, publicly acknowledging a previously unreported incident.

FILE PHOTO: A Nigerien soldier walks out of a house that residents say a Boko Haram militant had forcefully seized and occupied in Damasak March 24, 2015. REUTERS/Joe Penney/File Photo

The fight took place two months after four American and four local soldiers were killed in Niger, and could prompt further questions about the extent of the little-reported U.S. mission in West Africa.

“During a mission in the Lake Chad Basin region the morning of Dec. 6, a combined force of Nigerien and U.S. military members came under fire from a formation of violent extremists,” United States Africa Command (Africom) said in a statement.

“We assessed 11 enemy killed in action, including two wearing suicide vests, and one weapons cache destroyed during this mission,” it said, adding that “no U.S. or Nigerien forces were killed or wounded during the attack.”

Niger’s defence ministry did not immediately respond to a request for comment.

A military source said the decision to issue the statement was taken after the New York Times learned the incident in an unclassified report the White House submitted to Congress this week.

The deaths of the four Americans in an ambush in Niger in October brought international and domestic attention to the U.S. mission to combat Islamist militants in the Sahel, the arid region south of the Sahara.

A regional branch of Islamic State claimed responsibility for that attack, which also led to a political fight between President Donald Trump and a congresswoman who accused him of speaking insensitively to the widow of one of those killed.

Southeastern Niger around Lake Chad, which borders Nigeria, Chad and Cameroon, is an area where militants from Boko Haram have been particularly active. The Islamist group has gradually split into two factions, one of which has pledged allegiance to Syria-based Islamic State.

This faction does not have any known links to Islamic State in the Greater Sahara, the group on the western side of Niger that launched October’s deadly ambush.

After the deaths of the Green Berets, the Pentagon was adamant that operation was not a kill-or-capture mission, but Nigerien officials insisted it was.

Anticipating similar questions about the December mission, Africom said: “there was no aspect of this mission focused on pursuing enemy militants,” adding: “the purpose of this mission was to set the conditions for future partner-led operations against violent extremist organizations”.

The United States has 800 soldiers operating in the largely desert nation, more than France, which has 4,000 in the wider Sahel tackling the Islamist militancy.