"Boko Haram attacked a base of the Mixed Multinational Force on the island of Darak (on Lake Chad) at around 2200 (2100 GMT)" yesterday, said the source, who asked not to be named. He said six soldiers had been killed.
Boko Haram frequently travel around the many islands of Lake Chad, which borders northeast Nigeria, the far north of Cameroon, Niger and Chad.
A civilian from a vigilante group working with security forces to combat Boko Haram incursions was also killed, another security source said.
Cameroon has battled the extremist group since 2014 and has joined Chad, Nigeria, Niger and Benin in setting up a joint force against Boko Haram.
The regional counter-offensive has helped the Nigerian military retake swathes of territory from the insurgents, but the jihadist group still poses a security threat to civilians.
Also, yesterday evening, a would-be suicide bomber was shot dead before detonating her device as she sped on a motorbike towards a military roadblock near the Cameroonian border town of Kolofata.