Nigeria's armed group Boko Haram has pledged allegiance to Islamic State of Iraq and the Levant (ISIL), Reuters news agency reports quoting the monitoring group SITE.

The pledge, attributed to Boko Haram leader Abubakar Shekau, was made in an audio recording posted on the group's Twitter page on Saturday, but it could not be immediately verified.

"We announce our allegiance to the Caliph ... and will hear and obey in times of difficulty and prosperity," SITE quoted Boko Haram as saying in a video purporting to be from the Nigerian rebel group.

The video 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 ISIL, which controls large swathes of territories in Iraq and Syria.

Baghdadi has already accepted pledges of allegiance from other armed groups in the Middle East, Afghanistan, Pakistan and North Africa.

Speaking to Al Jazeera, Aliyu Musa, Boko Haram expert, said: "It's not the first time Abubakar Shekau has done this - remember, when they started taking parts of Nigeria last year, he did proclaim support for ISIL.

"We've always know that they have connections with al-Qaeda and al-Shabab and others, but now the government should be more aware they are connecting with violent groups around the world."

Boko Haram has been waging a six-year military campaign to carve out an Islamic state in northern Nigeria.

On Saturday, three bomb blasts killed more than 50 people and wounded scores of others in Maiduguri in the worst attacks there since Boko Haram fighters tried to seize the northeastern city in two major assaults earlier this year.

The suicide blasts occurred over four hours in locations from a busy fish market to a crowded bus station, Police Commissioner Clement Adoda said.

In the deadliest blast, 18 people died when a suicide bomber blew himself up in a tricycle taxi at the entrance to the bustling Baga fish market, police said.

A fifth explosion from a car bomb at a military checkpoint 75km outside Maiduguri wounded a soldier and two members of a civilian self-defence unit.

The bomber apparently wanted to reach the city, a police officer said on condition of anonymity.