Cameroon's army has killed 27 members of the Nigerian Islamist group Boko Haram near a northern town, state radio said on Wednesday, in a sign of the growing cross-border threat the militants pose.

The Boko Haram fighters crossed the border into Cameroon earlier this week after attacking a military base and police station in Nigeria and apparently sending some 480 Nigerian troops retreating across the frontier.

"Cameroon soldiers have killed 27 Boko Haram elements during an attack in a locality near Fotokol in the far north," state radio CRTV said, adding that the deaths occurred on Monday and Tuesday. There was no word on any Cameroonian casualties.