MAIDUGURI, Nigeria (AFP) - Boko Haram pledged allegiance to the Islamic State in Iraq and Syria (ISIS) in an audio message published online on Saturday, after three bombs wreaked havoc in north-east Nigeria, killing 58 and wounding scores.
"We announce our allegiance to the Caliph of the Muslims, Ibrahim ibn Awad ibn Ibrahim al-Husseini al-Qurashi," Boko Haram leader Abubakar Shekau said, referring to ISIS leader Abu Bakr al-Baghdadi.
Shekau said the pledge was made because of religious duty to Islam and as it would "enrage the enemy of Allah".
The eight-minute speech, in which Shekau was not shown, was published on a Twitter account used by Boko Haram and subtitled in English, French and Arabic. Shekau has previously mentioned al-Baghdadi in video messages yet stopped short of pledging formal allegiance.
But there have been increasing signs that the Nigerian militants, whose six-year insurgency has claimed more than 13,000 lives and left 1.5 million people homeless, has been seeking a closer tie-up.
Not only did Shekau announce last year that the captured town of Gwoza in Borno state was part of a caliphate, but in recent weeks Boko Haram videos have increasingly resembled ISIS propaganda.
Nigeria's President Goodluck Jonathan, who has persistently blamed the violence on outside forces, last month claimed the country had intelligence on Boko Haram links to ISIS.
Sunni militant specialist Aaron Zelin, a senior fellow at the Washington Institute of Near East Policy, said it was difficult to assess the immediate effect of Shekau's statement.
"Definitely it will put an even bigger target on their back," he told AFP in an email exchange.
He added: "It's relevant because it highlights the resonance of the idea of the caliphate.
"For years there have been rumours of connections with AQIM (Al-Qaeda in the Islamic Maghreb) or Al-Shebab but there was never anything definitive... and now (Boko Haram) decides to do this overtly."
Mr Max Abrahms, from Northeastern University in Boston, Massachusetts, said the pledge of allegiance "made sense", as both groups were currently "in a position of relative organisational weakness".
ISIS, on the back foot in Iraq, has been looking to broaden its reach beyond the Middle East, particularly in places where there are power vacuums and existing militant groups, such as Libya.
Boko Haram has apparently been pummelled out of captured territory by the Nigerian army and its regional allies and has returned to its previous campaign of urban guerrilla warfare.
Militant fighters were this week reportedly amassing in Gwoza - which is generally considered to be the group's headquarters - possibly in preparation for a military offensive.
Nigeria's military on Saturday announced another success against Boko Haram, ousting them from Buni Yadi and Buni Gari in Yobe state after previously claiming the recapture of Marte in Borno state.
Mr Abrahms, a specialist in extremist groups, said that with ISIS and Boko Haram both "uninhibited" in terms of violence, they were "a natural ally".
But the apparently more formal link could attract more global attention to the insurgency.
"I think Shekau now has to be careful here as traditionally the international community has regarded the conflict as a civil war within Nigeria, perhaps an extended regional war in west Africa," he added.
With Boko Haram squeezed out of captured territory, security analysts have predicted a rise in bomb attacks in towns and cities, including to disrupt elections in three weeks' time.
On Saturday, a woman with explosives strapped to her body blew herself up at about 11.20am at Baga fish market in the Borno state capital, Maiduguri.
About an hour later another blast rocked the popular Monday Market, causing chaos as locals voiced anger at security forces, who struggled to control the scene.
Just after 1pm a third blast hit a used car lot which is attached to the busy Borno Express bus terminal.
There were indications that the second and third blasts were also carried out by suicide bombers but details were not immediately clear.
Borno's police commissioner Clement Adoda gave a toll of 58 dead "for the three locations" in Maiduguri and 139 wounded.
"Normalcy has been restored," he added, declining to give further details.
Mr Danlami Ajaokuta, a vigilante leader whose fighters have been working with the military across the north-east, said the fear of further attacks had prompted the closure of all businesses in Maiduguri.
Borno state's Justice Commissioner Kaka Shehu blamed Boko Haram and described it as a response to the defeats suffered by the insurgents in recent weeks.
"The terrorists are angry with the way they were sacked from towns and villages and are now venting their anger," he said.
Nigeria postponed its elections initially scheduled for February to March 28 after security chiefs said they needed more time to weaken Boko Haram.
While reported victories in the remote northeast may enable polling in areas previously controlled by the insurgents, rising unrest in Maiduguri is likely to raise fears as election day approaches.
Shekau has vowed to disrupt the vote and widespread attacks, especially near polling stations, could prove disastrous.