An estimated 100,000 fish have died in the northwest of Arkansas, the same state where up to 5000 dead birds recently fell from the sky.

The Arkansas Game and Fish Commission said it suspected disease was to blame for the death of the drum fish, which floated in the water and lined the banks of a 30-kilometre stretch of the Arkansas River near Ozark, about 200km northwest of Little Rock, CNN reported on Sunday.

Ozark is about 200km west of the town of Beebe, where game wardens were trying to find out why up to 5000 blackbirds fell from the sky just before midnight on New Year's Eve, CNN said.

Keith Stephens from the game and fish commission said fish kills occurred every year but the size of the latest one was unusual, and suggested some sort of disease was to blame.

"The fish kill only affected one species of fish," he told CNN.

"If it was from a pollutant, it would have affected all of the fish, not just drum fish."

A tugboat operator discovered the dead fish on Friday night, and fisheries officials collected some of the dying animals to conduct tests.

Meanwhile, game and fish commission ornithologist Karen Rowe said the blackbirds found in Beebe showed physical trauma, and she speculated that "the flock could have been hit by lightning or high-altitude hail", the Associated Press reported.

The AP said more than 1000 birds fell from the sky but CNN put the number at up to 5000.

The commission said that New Year's Eve revellers shooting off fireworks in the area could have startled the birds from their roost and caused them to die from stress.

The commission said on Saturday that it began receiving reports about the dead birds about 11.30pm local time the previous night.

The birds fell over a two-kilometre area, and an aerial survey indicated that no other dead birds were found outside of that area.