BOKO Haram has pledged allegiance to the Islamic State group, after repeated indications that it was seeking a formal tie-up and a series of Nigerian military successes against the rebels.

The Nigerian group’s leader Abubakar Shekau made the announcement in an audio message on Saturday night, describing it as a religious duty and saying it would “enrage the enemy of Allah”.

The declaration raises the possibility that Western powers, which have so far stayed out of direct military operations in northeast Nigeria, might be pulled into the conflict, which has left more than 13,000 people dead since 2009 but largely been seen as a “local” or regionalissue.

“Operating as an IS proxy could draw Western efforts against its operations, particularly from France, which already runs a multi-country anti-terrorist campaign in the west and central African region,” said Ryan Cummings, from risk consultants Red24.

Extremists from Libya to Pakistan have previously pledged support or allegiance to IS leader Abu Bakr al-Baghdadi while the group itself last month expanded its reach into lawless Libya.

There have been growing indications that Boko Haram was looking to formally align itself with IS group, starting last year when Shekau declared the captured town of Gwoza, in Borno state, part of a caliphate.

That followed a similar declaration by Baghdadi while both groups have captured swathes of territory.

In recent weeks, previously poorly produced Boko Haram videos took on the look and feel of more polished IS propaganda and were posted directly online, guaranteeing a wider audience.

IS was also reported to have received pledges of allegiance from militants in Nigeria.

Max Abrahms, a specialist in extremist groups, said Boko Haram and IS, which has yet to respond to the pledge, were in many ways a perfect match.

“Neither group (IS and Boko Haram) is inhibited in terms of violence. They’re a natural ally,” said Abrahms, from Northeastern University in Boston, Massachusetts.

Shekau may have attempted to tone down his violent rhetoric and improve Boko Haram’s public image among the wider jihadi movement but indiscriminate attacks have continued against civilian “soft” targets.