LAGOS (Reuters) - Nigeria’s army arrested Khalid al-Barnawi, leader of Islamist militant group Ansaru which is a splinter faction of Boko Haram that has been accused of kidnapping and killing Westerners, a military spokesman said on Monday.

But a security expert said al-Barnawi’s supporters had assured him that the militant leader, who the U.S. State Department named in 2012 as having ties to Boko Haram and al-Qaeda’s north African wing, had not been captured.

Defence spokesman Brigadier General Rabe Abubakar said al-Barnawi, who is thought to be in his late 30s, was arrested in Lokoja, the capital of the central state of Kogi, on Saturday.

Reuters has been unable to independently verify the details provided by the military which has not yet released photographic or video evidence, as it has with previous arrests.

“He has been arrested. We have made that giant stride,” said Abubakar, adding that some Boko Haram fighters had surrendered to Nigerian troops.

Muhammadu Buhari, who took office last May, has made it a priority of his presidency to defeat Islamist militancy in Africa’s most populace nation.

The arrest of al-Barnawi, if independently confirmed, would be significant as jihadists have been pushed out of northeastern areas they once controlled and conflicting messages on social media suggest internal schisms.

But Nigerian security analyst Fulan Nasrullah expressed doubts that he had indeed been captured.

“Khalid’s people and I have spoken and they have said that he is free and was not captured, whether in Lokoja or anywhere else,” he said.

“They have killed seven different people at seven different times thinking they were Khalid al-Barnawi. They have no photos of him, nor do they know any concrete information about him,” he added.

The defense spokesman could not immediately be reached to respond to Nasrullah’s comments.

The United States has put al-Barnawi and two other Nigerian militants on the blacklist of “foreign terrorists”.

Britain also put Ansaru on its official “terrorist group” list, saying the group was aligned with al-Qaeda and was behind the kidnapping of a British national and a Italian who were killed in 2012 during a failed rescue attempt. In 2012 Ansaru claimed responsibility for the kidnapping of a French engineer.

Reports of the Ansaru leader’s arrest come amid developments that suggest the Islamist militant movement in Nigeria may be losing momentum.

Boko Haram leader Abubakar Shekau appeared in a video circulated last month in which he seemed to suggest he was ailing and Boko Haram was losing its effectiveness. But another video emerged last week saying there would be no surrender.

Boko Haram controlled a swathe of land in northeast Nigeria around the size of Belgium at the start of last year but was pushed out by Nigerian troops, aided by soldiers from neighboring countries.

It has since resorted to attacks on public places, such as markets and places of worship.