MAIDUGURI, Nigeria (Reuters) - Islamist militants and Nigerian troops have been fighting around a fishing town on Lake Chad as the insurgents step up attacks in the run-up to a presidential election in which security is a campaign issue, the army and residents said.

The Nigerian army said in a statement the militants attacked a military base in Baga, in eastern Borno state, on Wednesday evening. The borders of Nigeria, Niger, Chad and Cameroon meet on the lake.

Residents said 10 people had been killed and clashes were still going on as the militants tried to capture the town.

But Nigerian army spokesman Sani Usman said: “We are clearing and mopping up.”

The only military death was a member of the navy, which supported army troops along with the air force, Usman said.

Baga was the scene of mass killings by Boko Haram militants in 2015 when hundreds, possibly thousands, of people died and much of the town was destroyed. At the time Boko Haram held territory around the size of Belgium in northeast Nigeria but were later beaten back.

This attack, however, was the latest in a string of strikes on military bases in the northeast. President Muhammadu Buhari’s security record has become a campaign issue ahead of an election in February in which he will seek a second term.

The Boko Haram insurgency, which Buhari promised to end when he took office in 2015, aims to create an Islamic state in northeast Nigeria. It has killed about 30,000 people and forced about 2.7 million to flee their homes since 2009.

A faction of the group - Islamic State in West Africa (ISWA) - split from Boko Haram in 2016 and is now considered by security experts to be the stronger group.

Two military sources, speaking on condition of anonymity, said ISWA carried out the attack on Baga.

Two residents who fled the fighting in Baga said they saw the bodies of at least 10 people. Eight of the dead wore military uniforms, one said.

The Nigerian army said the military base attacked housed a unit of the Multinational Joint Task Force, which comprises of soldiers from Nigeria and neighboring Niger, Chad and Cameroon.

About 100 soldiers were killed in November by ISWA militants who attacked an army base in Metele, Borno state. That was the biggest loss of military life in the last few months.

Buhari’s opponents used the attack to question the former general’s handling of the insurgency.