A joint written statement with Nippon and the Cowlitz County Sheriff's Department said a tank containing "white liquor," a chemical solution of sodium hydroxide and sodium sulfide used in making paper pulp, had ruptured about 7.15am on Tuesday (local time).
While the tank was initially believed to hold about 302,830 litres (80,000 gallons), officials later said it contained closer to 3.4 million litres (900,000 gallons) of white liquor, with an estimated 340,687 litres (90,000 gallons) of material still inside the damaged tank.
Recovery efforts would resume on Wednesday at the site in the city of Longview in Cowlitz County, about 72km north of Portland, Oregon, as the tank remained unstable, Longview Fire Department Battalion Chief Matt Amos said during a media briefing on Tuesday night.
The injured, some critical, included eight employees of the facility as well as one firefighter, Amos said, adding that the firefighter had been treated and released.
Authorities had earlier said that multiple patients suffered from chemical burns.
Officials reiterated in the evening that the implosion posed "no direct threat to the surrounding community."
At the media briefing, Washington state Governor Bob Ferguson said: "It's difficult always to find the words at a time like this ... Our thoughts and our prayers are with everybody impacted by this tragedy."
PeaceHealth St John Medical Center in Longview told ABC News earlier in the day that six of the patients were in fair condition.
Nippon Paper Industries, Japan’s second-biggest paper manufacturer by sales, acquired the Longview plant from Seattle-based timber company Weyerhaeuser for $US225 million ($NZ382 million) and established the wholly-owned subsidiary Nippon Dynawave Packaging in 2016.
Japanese media reported that no injury was confirmed among Japanese employees, citing the Consulate-General of Japan in Seattle.
Meanwhile, in southern California, authorities have been monitoring an overheating industrial tank containing highly flammable methyl methacrylate.
The worst-case possibility of an explosion was ruled out on Monday at the GKN Aerospace facility in Garden Grove after a crack relieved some of the mounting pressure, officials said.
Orange County Fire Authority spokesman Greg Barta said in an update on Tuesday morning that the temperature in the tank was holding steady and that crews were working to ensure that people evacuated could get home as soon as possible.
