Four more girls kidnapped by Boko Haram militants last month escaped their captors, the education commissioner for Nigeria's Borno state said, leaving 219 still missing.

The girls were taking exams at a secondary school in the remote northeastern village of Chibok on 14 April when Islamist gunmen loaded 276 of them onto trucks and carted them off.

Fifty-three escaped shortly afterwards, say authorities in Borno state, which lies at the centre of the insurgency.

Education commissioner Musa Inuwa declined to give further details of the escape of the four girls.

The girls' abduction shone an international spotlight on the the militants, whose violent struggle for an Islamic state in northern Nigeria has killed thousands and turned them into the biggest threat to security in Africa's top oil-producing state.

From being a religious movement opposed to Western culture - Boko Haram means "Western education is a sin" in the northern Hausa language - the sect has emerged as a well-armed insurrection with a growing thirst for blood.

Earlier this week, Chief of Defence Staff Air Marshal Alex Badeh said the military knew where the abducted girls were but ruled out a rescue by force for fear of endangering them.

Most officials think any raid to rescue them would run a high risk that the girls would be killed by their captors as Boko Haram has repeatedly showned ruthlessness in targeting civilians.