Troops from Cameroon and Chad killed more than 250 Boko Haram militants in two days of heavy fighting in a major regional offensive against the Islamists from neighbouring Nigeria, their governments said on Wednesday.

Cameroon said its forces fought off an attack on one of its border towns early on Wednesday by Boko Haram, a group that has kidnapped hundreds and killed thousands in northern Nigeria and mounted increasingly bloody cross-border raids.

At least 50 militants and six soldiers died in the fighting in Fotokol, information minister Issa Tchiroma said. Cameroon's L'Oeil du Sahel newspaper said Boko Haram fighters raided mosques and houses in the town, slitting people's throats and killing dozens.

The reports could not be verified and Boko Haram does not give statements to the media.

The Sunni Muslim jihadist group is seen as the main security threat to Nigeria, Africa's biggest economy and top oil producer, ahead of a February 14 election. The African Union last week authorised a regional force of 7,500 troops to fight the militants.

Chad's army and state television said it had "completely wiped out" Boko Haram bases at Gambaru and Ngala in northern Nigeria on Tuesday, killing more than 200 militants. Nine Chadian soldiers died, they added.

Chadian TV footage on Wednesday showed jubilant Chadian soldiers cheering near to the bodies of what it said were two Boko Haram fighters.

Chad and Cameroon are deploying thousands of troops and Niger has reinforced its border against the militant group which has been fighting for five years to create an Islamist emirate.

But analysts say they are facing an uphill battle against a sect that has already seen off major offensives by Nigeria's military.

"Whether through intimidation, opportunism or genuine support for the radical Islamist cause, local communities continue to swell Boko Haram's ranks," said Roddy Barclay, senior Africa analyst at consultancy Control Risks.

Chad, reputed to have one of the region's best militaries, has carried out air strikes on insurgent positions over the last few days. Former colonial power France is also sending aircraft from its base in the Chadian capital to carry out surveillance missions in the border area.