Boko Haram's pledge of allegiance to the Islamic State group was a sign of weakness and a result of pressure on the jihadist militants by Nigeria and its allies, the government in Abuja said on Tuesday.

National security spokesman Mike Omeri called the pledge "an act of desperation and comes at a time when Boko Haram is suffering heavy losses."

The Islamists' leader Abubakar Shekau made the announcement in an audio message on Saturday night after months of indications that Boko Haram was seeking a formal tie-up.

Troops from Nigeria, Cameroon, Chad and Niger have claimed a series of successes against the militants since last month, pushing them out of captured territory in northeast Nigeria.

The operation is designed to secure and stabilize the region in time for Nigerian general elections, which were postponed by six weeks from February 14 to March 28.

Omeri said in a statement that the four-nation coalition force was "rooting out Boko Haram out of its strongholds and degrading its combat abilities."

"Boko Haram is on the way to being eliminated," he added.

"No foreign extremists can or will change this fact — as long as the Nigerian military continues to receive cooperation and commitment from its citizens and allies."

Boko Haram's seizure of territory in three northeast states from mid-2014 — a tactic also seen by ISIS in Syria and Iraq — raised fears of a loss of government control in the remote region.

Security analysts have said the pledge of allegiance to ISIS leader Abu Bakr al-Baghdadi may only have propaganda value in the short term but in the future could lead to closer links.

But Omeri said on Tuesday: "There will be no Islamic State in Nigeria, the only state that will exist is the united Federal Republic of Nigeria."