Islamic State jihadists ambushed a Nigerian military convoy, killing 15 soldiers and four militia fighters in the northeastern state of Borno on Thursday, security sources said in a statement over the weekend.

The 10-vehicle convoy was on its way to Gudumbali from the town of Kukawa in the violent Lake Chad region to participate in a military operation against the Islamic State when it came under fire, according to military sources. 

“We lost 15 soldiers and four civilian JTF (militia) in the terrorists’ ambush in the forest near Gudumbali,” an unnamed military officer said to the AFP. He added that 13 government fighters (10 army troops, three militia) were wounded in the fighting. 

“The casualties were brought to Maiduguri (regional capital), this afternoon,” the officer added.

The militia, who work with the Nigerian military in the fight against the insurgents, also released a statement. Their leader Umar Ari said that a militia leader in the area was among those killed.

“Four of our comrades were among the dead, including, Yusuf Baba-Idris, the head of Civilian JTF in Kukawa,” Ari said.

Nigeria’s security situation is tense. President Muhammadu Buhari has to deal with Nigeria’s many security challenges, including herder-farmer clashes and groups of bandits and kidnappers that prey on civilians in the northwest part of the country. Boko Haram’s insurgency adds another level to that pressure.

Nigeria’s National Security Adviser Babagana Monguno announced on Thursday that Nigeria will not negotiate with the terrorist group and that the government does not see negotiations as a means to deal with the worsening security situation in the country.