In a move seen as a propaganda boost to Islamic State, Boko Haram leader
Thousands of fighters from around the world have traveled to Syria to fight with Islamic State, some becoming suicide bombers. They include Australian teen Jake Bilardi, 18, who reportedly died in a suicide attack this week.
Islamic State's formal acceptance of the pledge Thursday was widely expected. Boko Haram's declaration was celebrated by Islamic State supporters, according to Twitter accounts purporting to be associated with the group. Photographs were circulated of fighters praising the move and sheep being slaughtered in celebration.
"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," Islamic State spokesman Abu Mohammad Adnani said in an audio message posted online.
Adnani called on Muslims who couldn't get to Syria to join Boko Haram's fight instead.
The pledge comes at a time when Boko Haram and Islamic State have reportedly suffered military setbacks.
Boko Haram declared its own Islamic state last year, capturing a large swath of territory, but its ability to run a state and hold territory is in doubt after a regional military force has driven its militants from more than 30 towns in recent weeks.
The African Union has endorsed a force of 10,000 soldiers to fight Boko Haram.
With just over two weeks to go before Nigerian elections, President
In Syria, Islamic State has been targeted by airstrikes by a U.S.-led coalition as the militants battle Syrian army, other rebel groups and Kurdish forces. In Iraq, army forces and militias this week advanced into Tikrit, a militant stronghold and the hometown of former Iraqi President Saddam Hussein.
The acceptance of Boko Haram's pledge comes as Islamic State challenges Al Qaeda as the assumed leader of the Islamist militant movements.
Islamic State has threatened to attack Europe as part of its ideological goal of taking over Rome, the seat of the Roman Catholic Church. It has also issued threats against the U.S., France, Italy, Britain, Australia and Canada, which are members of the coalition carrying out airstrikes against the group.