Explosion hit the secretariat but no group has claimed responsibility for the blast, and no casualties have been recorded.
According to Premium Times, the blast levelled the facility, popularly known as Isoko House, hours after the union began its annual conference there.
Witnesses believe the incident was a coordinated attack.
As at the time of filing this report, no group has claimed responsibility for the blast, and no casualties have been recorded.
The oil-rich Niger Delta region has witnessed several attacks in the past seven months, as separate militant groups bomb oil installations on weekly basis.
The Niger Delta Avengers, who recently announced a temporary ceasefire, claimed responsibility for most of the attacks on oil facilities.