At least eight children were killed by attackers who stormed the Mother Francisca International Bilingual Academy in the city of Kumba, in southwestern Cameroon, with guns and machete, according to the United Nations (UN).
It said a dozen more were wounded in the attack which occurred at around midday on Saturday.
They reportedly arrived on motorbikes and in civilian clothes.
“At least eight children were killed as a result of gunshots and attack with machetes. Another twelve were wounded and taken to local hospitals,” the UN’s Office for the Coordination of Humanitarian Affairs (OCHA) said in a statement.
The UN described the attack as “the worst atrocity” since the resumption of the school year earlier this month.
“They found the children in class and they opened fire on them,” city sub-prefect Ali Anougou told Reuters.
Local education official, Ahhim Abanaw Obase, confirmed six deaths of children aged between 12 and 14, and added that another eight had been taken to hospital.
African Union Commission Chair Moussa Faki Mahamat said: “There are no words of grief nor condemnation strong enough to articulate my full horror at the brutal attack targeting primary schoolchildren … as they sat learning in their classroom.”
It was not clear if the attack was linked to a continuing struggle between government forces and armed groups in the English-speaking west seeking to form a breakaway state.
Anglophone separatists have imposed curfews and closed schools as part of their protest against President Paul Biya’s government.
According to Reuters, Anougou and another official blamed Saturday’s attack on the separatists but did not offer evidence.