Nigerian troops have killed 29 suspected Islamist militants in two days of fighting in the northeast, where President Goodluck Jonathan's forces are trying to stamp out Boko Haram's four-year-old insurgency, the military said on Saturday.

A raid late on Friday targeted a Boko Haram base in the town of Bita in the Gwoza hills along the Cameroonian border, where the Islamists have set up camp after being pushed out of semi-desert areas further north.

"During the encounter, 20 Boko Haram terrorists were killed, scores of vehicles and 50 motorcycles were destroyed. A soldier was killed and three other soldiers were wounded in the operation," said Lieutenant Colonel Muhammad Dole, a spokesman for Nigerian forces in the northeasterly Borno state.

"We are closing in on the insurgents."

In a separate shootout on Thursday in the Damboa region of the same state, the military said it had killed nine militants.

Boko Haram has killed thousands during its struggle to try to impose Sharia or Islamic law on religiously mixed Nigeria.

It is seen as the main security threat to Africa's top energy producer and Western powers fear that it could export militancy as it expands its ties with al-Qaeda-linked Islamists.

That risk was highlighted on Thursday when gunmen kidnapped a French priest working in northern Cameroon, in an area where Boko Haram is known to operate. No one has yet claimed responsibility.

The United States formally designated Boko Haram and the splinter group Ansaru as foreign terrorists on Wednesday, making it a crime to materially aid them.

Since Jonathan declared a state of emergency in northeast Nigeria in May, deploying extra troops to try to flush Boko Haram out, there has been a surge in attacks on civilians, with hundreds killed in recent weeks.

Rights groups such as Amnesty International say hundreds of Boko Haram suspects have also died in military captivity. Nigeria's military denies abusing captives.