The blast occurred in Nkambe, a town in the region, as school children were taking part in activities to mark National Youth Day, which has been celebrated in the country on Feb. 11 since 1966.

Many of those affected were school children.

Health workers were "working very hard" to treat many of the children who sustained "very serious" injuries, a security source in the region told Xinhua.

"The death toll could increase. It is a disturbing situation but our brave soldiers have beefed up security to track down the terrorists who committed this abominable act," the source said.

Authorities were yet to make an official statement.

Separatists had imposed a three-day lockdown to disrupt the activities in the regions where they have been clashing with government forces since 2017 in a bid to create an independent nation they call "Ambazonia."