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ATLANTA, May 3 (Reuters) - One contract worker was killed and 17 others injured when a boiler exploded on Saturday at an International Paper Co <IP.N> mill plant in western Mississippi, the company said.

The dead and injured were contract employees working at the International Paper Vicksburg plant and everyone had been accounted for, the company said.

"During the start-up of the mill's recovery boiler, there was an explosion causing injuries and significant damage," the company said in a statement.

"The mill has been undergoing its annual maintenance outage and was in the process of starting back up," it said, adding no International Paper employees were hurt in the blast at one of two boilers that power the plant.

The cause of the explosion was being investigated, the company said.

"We have been given the all-clear on the parts of the mills that were unaffected by the explosion and have secured the recovery boiler and surrounding area," International Paper spokeswoman Amy Sawyer said several hours after the blast.

The fire department first responded to the blast at the plant, located in Warren County, close to Mississippi's border with Louisiana, according to media reports. (Writing by Matthew Bigg; Editing by Peter Cooney)

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