In recent months, ISWAP has ramped up attacks in the northeast, targeting security officials and abducting civilians at fake highway checkpoints.
The decade-long conflict in remote northeastern Nigeria has claimed 36,000 lives and displaced around two million from their homes, according to the United Nations.
The conflict has spilled into neighbouring Niger, Chad and Cameroon, prompting a regional military coalition to fight the jihadist groups.
Onomuakpokpo was leaving the building site where he went to supply sand when the gunmen in a White Toyota Camry opened fire on him.
The Delta State police command has confirmed his killing.
Details soon.