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Boko Haram militants have been waging a decade-long insurgency in northwest Nigeria, but the conflict has spilled into Lake Chad where Nigeria borders Niger, Chad and Cameroon.

Security sources said the seven Chadian soldiers and the guard were killed in an ambush on Friday in Mbomouga in Chad's Ngouboua area, and another 13 people were wounded.

"The Boko Haram forces lost six of their people and left behind two weapons," one security source said. 

Among the soldiers killed was a gendarme colonel, the source said. Another source said three army officers were killed in the attack.

Since 2018, Boko Haram has carried out at least nine attacks on Chad. But the militant group has stepped up attacks outside Nigeria after a period of calm last year.

Last month, militants killed four people in an attack on a Cameroonian island on Lake Chad and Boko Haram killed another 13 villagers in eastern Chad.

In March extremist Islamist militants killed at least 23 Chadian soldiers in an attack on an army post in the group's deadliest attack on the country's military.

Since 2015, troops from Cameroon, Chad, Niger and Nigeria have been grouped into a mixed, multi-national force in a bid to help fight Islamist militants.

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