Emergency responders were working on recovery efforts at Nippon Dynawave Packaging Company in Longview, the Longview Fire Department said on Tuesday.
Cowlitz Fire and Rescue Chief Scott Goldstein told a news conference it was unclear how many workers had been killed.
Asked how many remained missing, he replied: "We're not releasing that information".
Among those injured was a responding firefighter. Authorities said some victims had suffered burns or inhalation injuries, and that the severity of the injuries ranged from minor to critical.
There was no immediate threat to the public, they said.
The statement, issued more than four hours after the tank imploded at 7:15am said crews were continuing recovery operations and that no identifying information would be released about victims pending notification of relatives.
The facility, which employs about 1000 people, makes material for tissues, printing paper, cups, plates, cartons and other goods. It is located in an industrial zone shared by other timber, paper and chemical businesses.
The 303,000-litre tank that imploded was about 60 per cent full, Goldstein said.
It contained a chemical brew known as "white liquor," a corrosive substance that consists mainly of sodium hydroxide and sodium sulfide and is used to break down wood to make a durable paper used in packaging, shopping bags and other products.
It was too early to determine the cause of the implosion, Goldstein said.
In a written statement, US Senator Patty Murray of Washington called the implosion an "absolute tragedy".
"My heart is with everyone who lost a loved one today - as well as the injured workers," she said.
Mike Gorsuch, battalion chief with the fire department in Longview, Washington, described it as a "mass casualty scene".
First responders had decontaminated patients and taken them to hospitals in Longview and Vancouver, Washington.
About 40 firefighters and paramedics had responded, along with a regional hazmat team, Gorsuch said.
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