No group immediately claimed responsibility for the attack, but Boko Haram and rival Islamic State in West Africa have often carried out attacks in Nigeria's Borno state
Dozens of Islamist militants came in the night on horseback and abducted the girls from two villages in the Diffa region
The effort is part of a U.S. effort to counter violent extremists in the region, U.S. spokeswoman says
The April 2014 abduction by Boko Haram brought the extremist group's rampage in northern Nigeria to world attention
President Muhammadu Buhari praises troops for defeating terrorism group that has killed tens of thousands, but simultaneous attack shows fighting unlikely to end soon.
Purported release comes few months after video was published showing one of the girls begging Nigeria's government to exchange detained militants for the girls' freedom.
African terrorist group says some of the girls killed in airstrikes; captive pleads with Nigerian government to release detained Boko Haram members in exchange for the girls.
The group's leader had not fallen into line with ISIS instructions, including by ignoring calls for Boko Haram to stop using children as suicide bombers.
Another 60 people are left wounded after shooting, burning and explosions from three suicide bombers continued for nearly four hours on Saturday.
The forces will be conducting airborne intelligence, surveillance and reconnaissance operations.
Islamic militant group Boko Haram may be responsible.
Targeted village lies north of Maroua, where dozens of people were killed in a series of suicide bombings by the Nigerian Islamist group last month.
Militants shot dead 'many' people late on Wednesday in town in northeast Nigeria, according to several sources.
London-based human rights group says Nigerian army responsible for deaths of more than 8,000 people 'murdered, starved, suffocated, tortured to death.'
Many undergoing screening for various diseases and infections including HIV/AIDS, says UN.
Over 677 females have been declared rescued this week from group's stronghold in country's northeast.
Millions across Africa's most populous country, however, make it to the ballot box in safety in the most closely contested presidential race in the nation's history.
Town of Damasak was seized by the Islamist group last November, but recaptured on Saturday by troops from Niger and Chad.
The pledge of allegiance was attributed to Boko Haram leader Abubakar Shekau.
Iran VP: Technical roadblocks hampering nuclear deal eliminated; ISIS militants destroy ancient remains of 2,000-year-old Iraqi city of Hatra; Egypt investigating how foreign tourists filmed pornographic video near Giza pyramids.
There were two explosions at two markets and a third blast at a bus station in Maiduguri, witnesses and the local government said.
'Weapons and equipment were also captured and some destroyed,' defense spokesman Major-General Chris Olukolade said in a statement.
In run-up to Nigeria's general elections, scheduled for February 14 and then postponed to March 28 due to security concerns, Boko Haram has stepped up brutal attacks.
Extremists are said to be attacking Maiduguri, the birthplace of the extremist movement, from four fronts and are within 10 miles of the city.
Terrorist group has been increasing its attacks as Nigeria prepares for national elections.
Nigerians wanted to buy retired Cobra attack helicopters last summer to fight Boko Haram.
Nigerian military repels multiple attacks on Borno state capital Maiduguri but Islamist militants capture another Borno town.
Government spokesman says militants released young men, women and children who were kidnapped on January 6.
Amnesty International says as many as 3,700 homes and business were destroyed in the January 3 attack on the town of Baga.
Boko Haram's leaders talk about reviving one of the West African Islamic empires that for centuries prospered, but they demonstrate little evidence of state building.