The army in Nigeria says a senior officer has been killed by Islamic State militants in north-eastern Borno state.

Brigadier General Dzarma Zirkusu and three other soldiers died in an ambush by the West African branch of Islamic State, known as Iswap.

It's reported the army unit was on its way to reinforce troops engaged in a fierce battle with the insurgents.

The military said the fighting erupted after Iswap fighters drove into Askira town in 12 armoured vehicles, burning down houses, shops and a school.

Islamists have waged a devastating insurgency in the country's north-east for more than a decade.

President Muhammadu Buhari has expressed his sadness at the deaths of the soldiers, saying they paid the ultimate sacrifice to protect fellow Nigerians from terror.