(Adds details on fighting, gains by Nigerian army)
By Madjiasra Nako and Beau-Bernard Fonka Mutta
N‘DJAMENA/YAOUNDE, March 12 (Reuters) - Much of the Chadian force fighting Boko Haram in Nigeria has withdrawn to Cameroon and is deploying further south, Chadian soldiers and a Cameroonian military official said on Thursday, signalling the potential start of a new offensive.
Chad’s military has spearheaded an operation by Nigeria’s neighbours against the Islamist militant group that has killed thousands in northeast Nigeria and in recent months mounted an increasing number of cross-border raids.
Nigeria has simultaneously launched an offensive to retake a series of towns with the backing of hundreds of mercenaries from South Africa and the former Soviet Union in a bid to secure towns ahead of a rescheduled March 28 election.
Chadian soldiers, who asked not to be identified, said troops operating in Nigeria had pulled back from their forward base of Gambaru to Fotokol, a town on the Cameroon side of the border.
“We don’t know the reason for the withdrawal. We just received the orders,” one of them said, adding that the force was now heading south to the border town of Banki.
Banki lies on the main road from Cameroon to the Nigerian town of Bama, the second-biggest town in Borno state. Security sources in Nigeria said fierce fighting was on-going to retake Bama, which has been held by Boko Haram since late last year.
A spokesman for the Chadian military was not immediately available for comment. A Cameroonian military source confirmed Chadian troops had crossed back into Cameroon and were headed south but gave no further details.
Chadian troops last week pushed the furthest they have gone into Nigeria when they freed Dikwa, a town at a major crossroad some 80 km (50 miles) east of Maiduguri, the capital of Borno.
Since then, Nigeria asked Chadian troops to leave and deployed its own troops to the town, Chadian officers said.
Nigeria’s defence headquarters said on Thursday that their army had cleared the last Boko Haram camps in Adamawa state, which borders Borno.
Chadian troops are also battling Boko Haram to the north along the long, porous border with Niger.
A Niger military source said about 20 Boko Haram militants were arrested in Damasak on Thursday, highlighting how fighters were still present in the zone despite reports from Niger officers that they had scattered them this week.
Mistrust and rivalries have hamstrung coordination between the regional armies, which are in the process of planning and seeking United Nations backing for a joint 10,000-strong force to defeat Boko Haram.
Chadian officials complain that Nigeria has repeatedly prevented them from advancing despite Chad having scored several victories against the militants, who recently pledged allegiance to Islamic State and aim to carve out a caliphate in Nigeria’s northeast.
Nigeria’s military denies any lack of cooperation and says it has not been credited enough for its own gains against Boko Haram.
With Nigerian President Goodluck Jonathan seeking reelection at polls on March 28, his opponent Muhammadu Buhari has lambasted his government for relying on Chadian intervention to tackle Boko Haram. (Additional reporting by Julia Payne in Abuja, Lanre Ola in Maiduguri and Abdoulaye Massalaki in Niamey; Writing by David Lewis; Editing by Hugh Lawson)