Nigerian security forces have uncovered a Boko Haram bomb factory this week, after regaining control over a town seized by the extremist group. The factory was discovered inside a fertilizer company in Buni Yado town in Yobe state.

The Boko Haram fighters had been in control over Buni Yado since August. Nigerian troops had only regained control over the town last week, after months of ground battle against the militants.

The troops found suicide bomber vests and improvised explosive devices. “Troops are still evacuating the materials … a large quantity of improvised explosive devices, including those commonly used by suicide bombers, were recovered from the site,” the military said in a statement.

Four soldiers were killed during the offensives in Buni Yado. The troops had difficulty advancing to the town after militants planted explosive devices along the highway on their way out.

The discovery of the bomb factory came after Boko Haram and the ISIS announced an alliance.

In an audio message, the ISIS accepted Boko Haram’s pledge of allegiance, saying their establishment of a caliphate has extended to West Africa. The ISIS even urged Muslims in Western Africa to join the Boko Haram.

Days before that, Boko Haram’s leader, Abubakar Shekau, had also posted a video message in its social media accounts, announcing its pledge of allegiance to the Islamic state.

The video message of Boko Haram had been verified by US-based agency SITE, that monitors activities of extremist organizations, while that of the ISIS had not been authenticated.

Observers had said, Boko Haram had shifted its tactics as pressure mounts from African nations which forged alliance in a bid to bring them down.

An 8,000-strong army from Nigeria, Chad, Cameroon and Niger launched offensives against the insurgents and vowed to wipe out all Boko Haram camps.

Boko Haram had been sowing terror in northern Nigeria since 2009, attacking police, schools, churches, mosques and civilians. It has also kidnapped students, including more than 200 schoolgirls who have gone missing since April.