Nigerian militant group Boko Haram has pledged allegiance to Islamic State (IS), according to an audio statement.

The message, which has not been verified, was posted on Boko Haram's Twitter account and is believed to be by the group's leader, Abubakar Shekau.

Boko Haram began a military campaign to impose Islamic rule in northern Nigeria in 2009. The conflict has since spread to neighbouring states.

It would be the latest in a series of groups to swear allegiance to IS.

In the past Boko Haram is thought to have had links with al-Qaeda.

IS took control of large swathes of territory in eastern Syria and across northern and western Iraq last year.

The group aims to establish a "caliphate", a state ruled by a single political and religious leader according to Islamic law, or Sharia. Its leader, Abu Bakr al-Baghdadi, is known to his followers as Caliph Ibrahim.

In the audio message posted on Saturday, the Boko Haram leader purportedly said: "We announce our allegiance to the caliph... and will hear and obey in times of difficulty and prosperity.

"We call on Muslims everywhere to pledge allegiance to the caliph."

Boko Haram's insurgency has threatened Nigeria's territorial integrity and triggered a humanitarian crisis.

It has carried out frequent bombings that have left thousands dead in Nigeria's northeast and has also attacked targets in the capital, Abuja.

On Saturday, at least five blasts including several suicide bombings in the northeastern Nigerian city of Maiduguri left at least 50 people dead.

The targets included two busy markets and a bus terminal in the city. Eyewitnesses have said at least two of the suicide bombers were women.

Boko Haram has not yet commented on the attack, but it has used suicide bombers in the past and Maiduguri is its former stronghold.

The Nigerian military and troops from neighbouring states have recently claimed a series of successes in their efforts to push back Boko Haram. Nigeria postponed national elections by six weeks until 28 March in order to have more time to try to improve security in the north.

IS has forged links with other militant groups across North Africa and the Arabian Peninsula.

In November Abu-Bakr al-Baghdadi accepted pledges of allegiance from jihadists in Egypt, Libya, Algeria, Yemen and Saudi Arabia.

In January, militants in Afghanistan and Pakistan announced that they were forming an IS "province".

IS is distinguished by the brutality of its tactics, which include mass killings and abductions of members of religious and ethnic minorities, as well as the beheadings of soldiers and journalists.

Since last year, a US-led alliance has tried to counter the group with air strikes in Iraq and Syria.

Boko Haram at a glance

Founded in 2002, initially focused on opposing Western-style education - Boko Haram means "Western education is forbidden" in the Hausa language

• Launched military operations in 2009 to create Islamic state
• Thousands killed, mostly in north-eastern Nigeria - has also attacked police and UN headquarters in capital, Abuja
• Abducted hundreds, including at least 200 schoolgirls
• Controls several north-eastern towns
• Launched attacks on neighbouring states