Nigeria has begun the "final onslaught" against Boko Haram, the country's national security spokesperson has said, after the armed group was thrown out of the strategic town of Bama.

On a visit to London, Mike Omeri said on Tuesday that "significant strategic military successes and gains" had been made against Boko Haram in recent weeks.

"Bama was retaken yesterday and we have Abadam, Gwoza and Askira as part of the remaining areas where we still have this presence," he said was quoted by AFP news agency as saying.

Abadam, Gwoza and Askira are also in Borno state, 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 in Iraq and the Levant (ISIL) group.

Boko Haram is fighting to form an Islamic state encompassing several states of northern Nigeria.

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.

Voting postponed

The operation not only aims at reducing the regional threat from the group, 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 effort 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."

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

On the subject of 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.