At least one person has been killed and nine others are still missing after a major chemical explosion at a paper mill in Washington, authorities said.

The explosion occurred at 07:15 PDT (15:15 GMT) at the Nippon Dynawave Packaging facility in Longview, 130 miles (210km) south of Seattle.

A further nine people have been injured in the spill, including 8 employees and one firefighter, the Longview Fire Department said Tuesday evening, adding that the tank remains unstable.

The explosion occurred due to a "rupture of a tank containing white liquor", the company said in a statement. White liquor is a highly corrosive chemical used in the paper-making process.

"Recovery efforts remain complex due to ongoing safety concerns at the site," the fire department said in an evening update.

"Currently, the tank remains unstable, creating hazardous conditions for emergency personnel," the statement continued. "Responders are continuing efforts to structurally reinforce and stabilise the site before additional recovery operations can safely proceed."

While investigators originally believed the tank contained about 80,000 gallons (300,000 litres) of white liquor, officials later said the tank held approximately 900,000 gallons (3,406,000 litres).

About 90,000 gallons (340,000 litres) "may remain inside the damaged tank", the fire department said.

Cowlitz Fire and Rescue Chief Scott Goldstein said during an earlier news conference that 10 people had been injured and transported to the hospital.

Nine of the people injured are employees at the factory, and one is a firefighter, Goldstein said at the time. Their injuries range from "critical severe to minor", and include burn and inhalation injuries. The company statement said there were "multiple critical injuries".

The scene is stable, the chief said, but the public should stay away from the area as firefighting efforts continue. He said the fire does not pose any threat to the larger community.

"The scene remains in the recovery phase as emergency responders continue operations," the Longview Fire Department said in a statement.

"No identifying information regarding injured or deceased individuals will be released at this time pending notification of family members."

White liquor is an alkaline chemical containing sodium hydroxide and sodium sulfide.

Washington Governor Bob Ferguson said state ecology workers have been sent to the site to assist local officials.

"I'm deeply saddened to hear that there have been fatalities," Ferguson said in a statement.

"My thoughts are with the workers and their families, and with the first responders."

According to local media, the Nippon Dynawave Packaging facility was also the scene of a major fire in July 2023, when piles of wood at the site burned for days.

The plant makes tissues, printer paper, cups, plates, cartons, and other goods, according to CBS, the BBC's US partner. It employs 1,000 people, according to the Washington State Department of Ecology.

The explosion comes as officials in California say a chemical tank that threatened to explode has now been rendered safe.

The tank in Orange County contains a chemical used to make plastic, and its possible explosion prompted an evacuation order affecting 50,000 people.

Officials in Washington say no evacuation order is needed.