Cameroon security forces killed some 40 Boko Haram militants in clashes in the country's far north, state radio said on Sunday, shortly after the release of two Italian priests and a Canadian nun suspected to have been held by the Islamist group.

A presidency source confirmed the clashes, which took place west of the town of Kousseri, in the region bordering Nigeria and Chad.

Cameroon, which has been criticised by Nigeria for not doing enough to fight the Nigeria-based Boko Haram, deployed some 1,000 troops to the far north this week as it steps up the fight against the Islamist militants.