Beirut - The leader of the Islamic State militant group that controls parts of Syria and Iraq has accepted a pledge of allegiance from Nigerian Islamists Boko Haram, his spokesman said in an audio message transmitted on Thursday.
"Our caliph, God save him, has accepted the pledge of loyalty of our brothers of Boko Haram so we congratulate Muslims and our jihadi brothers in West Africa," spokesman Abu Mohammad al-Adnani said, referring to Islamic State leader Abu Bakr al-Baghdadi.
Islamic State, an ultra-hardline offshoot of al Qaeda, has declared a caliphate in captured territory in Iraq and Syria and has gained global notoriety for killing or kidnapping members of religious minorities and posting videos of its members killing Arab and Western hostages.
Boko Haram, which has killed thousands and kidnapped hundreds during a six-year campaign to carve out an Islamist state in northern Nigeria, pledged its allegiance to Baghdadi's group last week, highlighting increased coordination between jihadi movements across north Africa and the Middle East.
In the audio message, Adnani called on Muslims who could not join Islamic State in Syria and Iraq to enter combat in Africa instead. He also issued a threat to Jews and Christians.
"If you want to save your blood and money and live in safety from our swords ... you have two choices: either convert or pay jezyah," he said, referring to tax for non-Muslims under Islamic rule.
"(Otherwise) you will soon bite your fingers with remorse."
Adnani also played down reports of military setbacks for the group which is coming under attack from Iraqi and Kurdish forces as well as U.S.-led air strikes.
In Syria, Kurdish YPG fighters have cut an important supply route from territory controlled by Islamic State in Iraq.
"The State remains steadfast ... and is becoming stronger and continues to be victorious," Adnani said, describing gains claimed by its enemies as "fake".
"It is a mere taking back of a few villages in a war that is about attack and retreat," he said, in the group's first official reaction to the developments.