YOLA, Nigeria - Nigeria's Boko Haram Islamic extremists have overrun a remote military camp in the northeast, leaving 13 soldiers wounded and an unknown number missing, the army said Wednesday.

Monday's attack comes a week after one faction of Boko Haram released 21 of more than 200 schoolgirls kidnapped from northeastern Chibok town, and as Nigeria's government is negotiating for the release of another 83 of the girls abducted 2 1/2 years ago.

The attack on Gashigar, on the border with Niger, is the third reported attack on the military after months of a lull during which the Islamic extremists hit soft civilian targets.

Army spokesman Col. Sani Kukasheka Usman called the attack a "temporary setback" committed by "remnants of Boko Haram" that forced the soldiers to retreat. An operation is in progress to find the missing troopers and "clear the Boko Haram terrorists at the general area," his statement said.

It is believed the attack is by a splinter from Boko Haram that calls itself the West Africa Province of the Islamic State. The IS named a new caliph of its only franchise in sub-Saharan Africa in August, provoking a struggle with Boko Haram's longtime leader Abubakar Shekau. A battle of words on social media indicated the dispute is over Shekau's indiscriminate killing of Muslims.

The group loyal to Shekau negotiated — with the Swiss government and International Committee of the Red Cross acting as intermediaries for Nigeria's government — last Thursday's release of 21 Chibok girls, the first such negotiated settlement.

President Muhammadu Buhari, who flew to Germany the day the girls were set free, is scheduled to meet with them and their families later Wednesday, according to a social media message posted by the official account of Nigeria's presidency.

Boko Haram's 7-year-old Islamic uprising has killed more than 20,000 people, forced some 2.6 million from their homes and left tens of thousands facing famine-like conditions, according to aid agencies and the U.N.

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Faul reported from Johannesburg.

News from © The Associated Press, 2016