KATHMANDU, 8: 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.”
Analysis: Jim Muir, BBC News, Beirut
The announcement by the Boko Haram leader came in a well-produced audio message posted on the internet, with rolling translation from his Arabic to both French and English. Although it was not an elaborate video production like those put out by IS itself, it had many of the IS hallmarks, including an Islamic anthem at the beginning.
The step came as no surprise, given evidence in Boko Haram’s propaganda output of growing IS influence on the Nigerian movement, whose ideology and harsh practices mirror those of IS itself.
What it will add up to in practical terms, given Boko Haram’s local roots and the geographical distance between them, is hard to see. But from a propaganda point of view, it’s a boost for IS, whose presence on the ground in Iraq and Syria is generally stalled and on the defensive.
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 north-east and has also attacked targets in the capital, Abuja.
On Saturday, at least five blasts including several suicide bombings in the north-eastern 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 state