Four North Americans were kidnapped in Nigeria on Tuesday. AFP/Getty Images

The leader of Ansaru, a splinter group of the Islamic extremist organization Boko Haram in Nigeria, was arrested Sunday, an army spokesperson said, Agence-France Presse reported. Boko Haram has lost ground to the Nigerian military over the past several months as the group was weakened, and the reported arrest of Khalid al-Barnawi was seen as a victory for the government in its fight against domestic terror.

"Security agents made a breakthrough on Friday in the fight against terrorism by arresting Khalid al-Barnawi, the leader of Ansaru terrorist group in Lokoja," military spokesman Brigadier General Rabe Abubakar said, adding, "He is among those on top of the list of our wanted terrorists."

Al-Barnawi was one of three Nigerians on the U.S. State Department's list of terrorists, and he has led Ansaru since 2012 after Adam Kambar died in a raid. Al-Barnawi has ties to the militant group al Qaeda in the Maghreb, according to both Nigerian military intelligence and the U.S. State Department.

Boko Haram has been carrying out bombings, massacres and land grabs since 2009 and declared an Islamic caliphate in 2014. The group first came to international attention after they kidnapped more than 200 young schoolgirls in Borno state in the northeastern region of the country. Leaders from across the world, including the U.S. President Barack Obama and first lady Michelle Obama rallied to ensure the safety of the girls. Their fate is still unknown.

Boko Haram flag
Nigerian soldiers hold up a Boko Haram flag they seized after taking back the town of Damasak in Nigeria's Borno state, March 18, 2015. Reuters/Emmanuel Braun

The Nigerian military has claimed strong victories over Boko Haram in the past few months, while international diplomatic officials have voiced their skepticism of those claims. The group’s ability to carry out attacks has been diminished, but they are not entirely defeated, according to experts.

"I am skeptical," said John Campbell, former U.S. ambassador to Nigeria, NBC News reported, adding, "The lack of credibility of the security forces over the past five years makes me inherently mistrustful of anything they say."