A photo taken from video by Nigeria's Boko Haram terrorist network shows their leader Abubakar Shekau. Shekau has allegedly made a formal allegiance to the Islamic State on Saturday. (AP)

Nigeria’s militant Islamist group Boko Haram pledged allegiance on Saturday to the Islamic State, which rules a self-declared caliphate in parts of Iraq and Syria, according to an audio clip posted online.

The symbolic move highlights increased coordination between jihadist movements across North Africa and the Middle East and prompted an appeal from Nigeria’s government for greater international help in tackling the Boko Haram insurgency.

Boko Haram has killed thousands and kidnapped hundreds during its six-year campaign to carve out an Islamist state in northern Nigeria. In recent months it has increased raids into Cameroon, Chad and Niger.

“We announce our allegiance to the Caliph . . . and will hear and obey in times of difficulty and prosperity, in hardship and ease,” read an English-language translation of the audio broadcast in Arabic that purported to be from the Nigerian militant group. “We call upon Muslims everywhere to pledge allegiance to the Caliph.”

The pledge of allegiance was attributed to Boko Haram leader Abubakar Shekau.

The audio script identified the caliph as Ibrahim ibn Awad ibn Ibrahim al-Awad al-Qurashi, who is better known as Abu Bakr al-Baghdadi, the leader of the Islamic State and self-proclaimed caliph of the Muslim world. The audio “is confirming what we always thought. It’s sad, it’s bad,” said Nigerian government spokesman Mike Omeri.

“It’s why we were appealing to the international community,” he said. “Hopefully, the world will wake up to the disaster unfolding here.”

On Saturday, four bomb blasts killed at least 50 people in the northeastern Nigerian city of Maiduguri in the worst attacks there since Boko Haram militants tried to seize the city in two major assaults earlier this year.

The Islamic State’s Baghdadi has already accepted pledges of allegiance from other jihadist groups in the Middle East, ­Afghanistan, Pakistan and North Africa.

Analysts said Boko Haram’s move came as no surprise.

“Boko Haram has followed a trend that only led [us] to anticipate the release of this audio, mimicking Islamic State propaganda and approach to military methods, and calling its fighters soldiers of the caliphate,” said Laith Alkhouri, director of ­Middle East and North Africa research and jihadist threat ­intelligence at Flashpoint ­Partners. “The Islamic State, unlike al-Qaeda, did not seem to shun Shekau,” he said. “It accepted his thuggish persona and lack of Islamic knowledge.”

— Reuters