Nigerian Red Cross officials on Friday said the clashes between Ijaw and Itsekiri groups in the southern Nigerian city claimed many lives, besides injuring 1000 others.
"We have reason to believe there were very nearly 100 deaths," Nigerian Red Cross President Emmanuel Ijewere said.
"We believe there were over 1000 injured," he added.
The Red Cross had earlier confirmed only eight dead and 54 wounded.
The violence between the two groups has been worst in the volatile Niger Delta when an Ijaw revolt led to dozens of deaths and forced oil internationals to halt 40% of Nigeria's output.
The Ijaws say political power is unfairly skewed in favour of the Itsekiris. They also want a greater share of the region's oil wealth.
Nigeria ordered in hundreds of armed police into Warri on Wednesday.