A school in the Congolese capital of Brazzaville has been closed by the authorities after students lowered the national flag and hoisted a different one.

Local media described it as an unidentified black flag.

The government condemned the act by students at Technical High School as a violation of the symbol of the republic and a "serious act of unpatriotism".

The government will take the necessary measures "in response to the crime", Education Minister Ghislain Thierry Magessa Ebomé was quoted as saying.

On Monday it was announced the school will be shut until further notice, and its registration for state exams has been suspended.

Thirteen students have been arrested over the incident and handed over to investigating authorities.