YAOUNDE, June 30 (Reuters) - Guards at a maximum security prison in Cameroon's port city of Douala shot dead 16 inmates during an attempted break-out, state radio said on Monday.

It said the prisoners made the escape bid from the New Bell prison late on Sunday, cutting off electricity and then opening fire on warders with two automatic rifles.

"The warders on guard ... responded with fire for fire. By the time the sustained gunfire died down, 16 inmates were killed and three others sustained serious injuries," the radio said.

Two of the prison guards were slightly hurt.

Security forces had sealed off neighbourhoods in Douala close to the prison to recapture any other inmates who might have managed to escape, state radio said.

Douala's New Bell jail, built in 1933 for 800 inmates, holds over 3,000 detainees, about two-thirds of whom are still awaiting trial. There had been two previous escape attempts in the past seven months.

Human rights campaigners say Cameroon's prisons suffer from chronic overcrowding, poor sanitation conditions and disease, and cases of violence and torture are frequent.

A new penal code introduced in 2006 limits the time of pre-trial provisional detention to six months, renewable for another six, but the measure is still not being fully applied. (For full Reuters Africa coverage and to have your say on the top issues, visit: http://africa.reuters.com/) (Reporting by Tansa Muasa; Editing by Pascal Fletcher)

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