The surprise attack led to the death of the outnumbered soldiers who were shot in the creek.
According to a report by The Punch, the victims were attacked along the waterways of Ekeremor Local Government Area of the state, while they were returning from Letugbene community.
The militants had caught them by surprise and disarmed them before moving them to a location in the creeks where they were murdered in cold blood.
The civilian, a boat driver, was shot to death after jumping into the river to escape the ambush attack.
Chairman of the Central Zone of Ijaw Youths Council, Tare Porri, confirmed the killings to Agence France-Presse (AFP).
He said, "Four military officers were killed in cold blood alongside a civilian. Military officers went to Letugbene community and on their way back, some persons accosted them and killed all of them."
He also said that the attack was unprovoked.