The Nigerian military says 20 people have been killed in clashes between the army and members of the Boko Haram militant group in the northeast.

The latest attack by the extremist group came on Saturday morning at Damboa town in Borno state.

The militants “attacked worshippers in a mosque and killed five of them who came to perform their morning prayers," the military said in a statement on Sunday, quoting the army spokesman for the region, Captain Aliyu Ibrahim Danja.

"They also set ablaze the official residence and palace of the District Head along with some shops,” he added.

The militant group was then repulsed by the military forces.

"While they were unleashing their mayhem, troops under 7 Division engaged the terrorists, killing 15 in the process while others fled in disarray."

Violence has intensified in northeastern Nigeria since President Goodluck Jonathan ordered his security forces in May to crush Boko Haram's four-year-old rebellion.

The Nigerian forces have been raiding militant camps and launching air and ground attacks on suspected hideouts of the militants over the past two months.

The military's newly formed anti-insurgency division has been taking part in the operations.

Boko Haram says its aim is to topple the Nigerian government, which it accuses of being pro-Western.

The group has claimed responsibility for a number of deadly gun and bomb attacks in various parts of Nigeria since 2009.

Over the past four years, violence in the north of Africa’s most populous country has claimed the lives of 3,600 people, including killings by the security forces.

In May 2013, the Nigerian government imposed a state of emergency in three states in the northeast, saying Boko Haram had become a security threat in those states.