BAMAKO (Reuters) - Authorities in Mali have arrested a man considered to be one of the main Islamist insurgent leaders operating in the south of the country, a hub for the expansion of militancy in West Africa, defense and intelligence sources said on Thursday.

The intelligence services had monitored Souleymane Keita since the capture of two of his men last year. He is believed by the authorities to be an associate of Iyad Ag Ghali, head of the northern Mali Islamist group Ansar Dine.

“We’d been on his trail since then (the capture of his men). And it was recently, when he attempted to come back into Mali across the border from Mauritania, that he was arrested by our men,” said one intelligence officer, who asked not to be named.

According to the officer, Keita had expanded Ansar Dine’s operations in southern Mali. He heads a group known as the Katiba Khalid Ibn Walid, analysts say.

A senior Malian defense official also confirmed Keita’s capture.

Ansar Dine was among a number of Islamist militant groups, some of them with ties to al Qaeda, that seized Mali’s northern desert region in 2012.

A French-led military intervention drove them back a year later but violence is again rising in Mali, with insurgents increasingly striking further from their traditional desert strongholds.

Islamist fighters have launched a series of bold attacks on civilian targets in southern Mali and in neighboring nations since last year.

Most recently, 19 people were killed when gunmen stormed the Ivory Coast beach resort town of Grand Bassam on March 13 in an attack claimed by al Qaeda in the Islamic Maghreb, the group’s North African affiliate.