'Knife-wielding' teens charged in NSW jail

Police have foiled four teenage inmates who allegedly armed themselves with knives and tried to escape through an air conditioning vent at a northern NSW youth detention centre.

Police were called to the Acmena Juvenile Justice Centre in South Grafton about 8.15pm Saturday and found the youths attempting to access the vent, which was about three metres off the ground.

The inmates, aged between 15 and 17, have been charged with affray, possessing an offensive instrument in a place of detention and malicious damage and will appear in a children's court via video link on Sunday.

Police said no employees at the facility were injured.