Nigeria has begun the "final onslaught" against Boko Haram, the country's national security spokesman said on Tuesday, after the militants were ousted from the strategic town of Bama.

On a visit to London, Mike Omeri told AFP that "significant strategic military successes and gains" had been made against the Islamists in recent weeks.

"Bama (the second biggest town in Borno state) was retaken yesterday (Monday) and we have Abadam, Gwoza and Askira as part of the remaining areas where we still have this presence," he said.

Abadam, Gwoza and Askira are also in Borno, which has been worst hit by six years of violence and was under emergency rule from May 2013 to November last year with neighbouring Yobe and Adamawa.

The military announced that Adawama was "cleared" last Friday and that Yobe was retaken on Monday from Boko Haram, who have pledged allegiance to the Islamic State group in Syria and Iraq.

There was no independent verification of the claimed successes, which followed the deployment last month of troops from Cameroon, Chad and Niger, as well as foreign mercenaries.

The operation not only aims at reducing the regional threat from the militants, after several cross-border attacks in recent months, but also to secure the northeast for elections to be held.

Voting was initially scheduled for February 14 but was rescheduled to March 28 because of the counter-offensive on the grounds that soldiers would not be available to provide security on polling day.

Omeri refused to be drawn on when the insurgency would be declared over, although President Goodluck Jonathan said in an interview published last Wednesday that Borno would be free in three weeks.

"As for the other three areas (Abadam, Gwoza and Askira), help is coming," said Omeri, who announced last week that 36 towns had been recaptured from Boko Haram.

"Soldiers are still out there working hard and we're en route to the final onslaught because it has started already from Bama."

The insurgency has left more than 13,000 people dead since 2009 and forced some 1.5 million others to flee their homes.

But Omeri said that once the affected communities were free, "they will be advised to return home and continue with their lives".

- Voter security -

Jonathan is facing a stiff challenge from the main opposition candidate Muhammadu Buhari, a former miliary ruler who has attacked the president's record on tackling Boko Haram.

Buhari, who headed a military government for 20 months from December 1983, has accused Jonathan of consistently failing to provide leadership.

Chief among the retired general and his party's complaints have been an alleged lack of support and equipment provided to soldiers, which only recently seems to have been rectified.

The opposition has also voiced fears for the integrity of the overall result if the hundreds of thousands of people displaced by the violence in its northeast stronghold are disenfranchised.

Repeated bomb and suicide attacks in recent weeks have raised fears about the safety of polling stations.

But Omeri told a separate news conference: "We are confident there will be a level of security to enable citizens to vote."

Boko Haram were now "running with their tails between their legs", he added, indicating that the militants were being contained within the northeast.

"Boko Haram are not being pushed into neighbouring countries, we are pushing them to an area where we are finding a solution to their menace," he said

Reports have suggested that Boko Haram fighters were amassing in Gwoza, which Boko Haram leader Abubakar Shekau declared part of a caliphate last year and is considered the group's headquarters.

On private military contractors, including South Africans who have been seen alongside Nigerian troops in the northeast, Omeri denied reports that some have been fighting on the front line.

He maintained their presence was only for training purposes and no mercenaries were involved.