Nigerian authorities on Sunday confirmed that nine youths drowned during a riverside picnic in the northeastern state of Taraba, Xinhua reported.
Responders have recovered only seven bodies since the incident on Saturday, said the National Youth Service Corps (NYSC).
All the victims were members of the NYSC, an organization set up by the Nigerian government.
Florence Yaakugh, a state coordinator of the NYSC, told reporters that 22 corps members were on a picnic at the River Mayo-Selbe in Gashaka area of Taraba when the incident occurred. The victims were on compulsory national service by young graduates.
Rescue operation is still ongoing to recover the remaining two bodies, Yaakugh said, describing the incident as "sad and most unfortunate."
The police said a preliminary investigation has shown that while the youths were swimming, there was a sudden upsurge in the volume of water in the river, which carried the victims away.