Nigerian troops have repelled simultaneous assaults by Islamist militants on two military bases in Borno state, leaving an unspecified number of soldiers dead, the army said on Monday, in some of the fiercest clashes reported in the northeast this year.

Borno, the epicentre of Nigeria's 17-year Islamist insurgency, has seen Boko Haram and Islamic State West Africa Province (ISWAP) fighters intensify attacks on both troops and civilians.

The weekend's coordinated assaults targeted Pulka, near the Cameroon border, and Mandaragirau in southern Borno, both long-contested fronts in the fight against Islamist militants.

The military said the failed attacks highlight mounting pressure on the insurgents.

President Donald Trump ordered airstrikes on Islamist militant targets in Nigeria's northwest in December and said there could be more US military action there.

Military spokesperson Sani Uba said ISWAP launched a multi-pronged assault using gun trucks and motorcycles in Mandaragirau, but soldiers, who had anticipated the strike, met them with heavy fire and prevented any breach.

Some soldiers and Civilian Joint Task Force members were killed in action, and several wounded personnel were airlifted out for treatment, Uba said, without providing further details.

In the second attack, Boko Haram and ISWAP fighters pushed into a troops' position in Pulka late Saturday, briefly breaching the perimeter and destroying accommodation and some equipment.

Troops held their ground and, with reinforcements, drove the militants back, Uba said.

The military said no troops were harmed in the Pulka assault, but credible battlefield intelligence indicated the insurgents took heavy losses, including the reported death of senior commander Abou Aisha from a gunshot wound to the neck.

Follow-up operations uncovered blood trails, shallow graves and abandoned equipment, Uba said.