There was a loud explosion in the hydrogen cracker plant of the refinery at 6:35 pm and a fire broke out which could be seen from a distance of 15 to 20 km.
Company fire tenders brought the blaze under control around 9 pm, sources said but could not give the number.
The plant was shut down immediately as part of the disaster management drill of the company.
A senior NRL official told PTI that there was no casualty or injury as the plant was automated and there was minimum deployment of human resources in the area.
ULFA's anti-talk faction claimed responsibility for the fire which was dismissed as 'unlikely' by senior police officials.
In an email to PTI, ULFA's central publicity committee member Joy Asom claimed that the outfit had carried out the blast with a 'Magnetic Time Device' on the occasion of its Raising Day.
Senior police officials, however, said "it is highly unlikely that it is an act of sabotage as the fire broke out in the middle of the plant area probably due to a technical fault.
The ULFA claimed that it had used the same device earlier in the Duliajan and Guwahati refineries last year but these too had been debunked by both police and refinery authorities since both were caused by technical problems.
The cause of the fire and the extent of the damage is yet to be ascertained.
Senior police and civil officials were at the scene.