A chemical tank ruptured at a facility in Washington state, resulting in multiple critical injuries and an unknown number of fatalities, authorities said.

The “hazardous materials incident” was reported Tuesday morning at the Nippon Dynawave Packaging, a pulp and paper mill in Longview, fire authorities said.

A tank containing white liquor ruptured at approximately 7:15 a.m., according to a joint statement from local authorities and Nippon Dynawave Packaging.

Authorities initially referred to the incident as a chemical explosion and then an implosion, before referring to it as a rupture.

“The incident is stable, but is in the recovery phase,” Longview Fire Battalion Chief Mike Gorsuch said during a press briefing Tuesday, calling it “tragic.”

“Recovery efforts remain underway at the facility,” he said.

The number of fatalities is undetermined at this time, Cowlitz 2 Fire & Rescue Chief Scott Goldstein said during the press briefing.

A number of personnel are missing, Goldstein said, declining to confirm how many.

Nine people at the facility suffered injuries, including chemical burns, and were transported by ambulance to area hospitals, authorities said. The injuries ranged from critical to minor, according to Goldstein.

One firefighter was also injured in the incident and has since been treated and released from a nearby hospital, according to Gorsuch.

Patients with traumatic injuries are being treated at nearby medical facilities, while patients experiencing burns and exposure are being sent to other hospitals around the area, including in Portland, which are considered burn centers, Rick Graves, the spokesperson for Portland Fire and Rescue in Oregon, told ABC News.

PeaceHealth St. John Medical Center in Longview told ABC News it received nine patients from the incident — including one deceased. Six are in fair condition, and two other patients have been transferred, it said.

Legacy Health confirmed to ABC News it is treating patients in the incident, including at the Legacy Oregon Burn Center in Portland, though did not go into further detail.

Information on those killed and injured in the incident is being held pending next-of-kin notifications, authorities said.

The 80,000-gallon tank was about 60% full at the time of the rupture, according to Goldstein. White liquor is a chemical mixture of sodium hydroxide, sodium sulfide and disodium carbonate used in the paper-making process, he said.

The cause of the rupture is unknown at this time, Goldstein said.

“We’re dealing with life safety concerns and incident stabilization at this point,” he said. “That will come in the hours and days to come.”

There is no immediate threat to the public, authorities said.

“The scene remains in the recovery phase as emergency responders continue operations,” the joint statement said. “Responding agencies continue coordinating closely with facility personnel and partner agencies as the investigation and recovery efforts continue.”

Residents were urged to avoid the area amid the active emergency response.

The Nippon facility is located on the Washington-Oregon border near the Columbia River. The kraft pulp and paper mill and liquid packaging plant employs nearly 1,000 people, according to the Washington Department of Ecology.

ABC News’ Trevor Ault and Alyssa Pone contributed to this report.

This is a developing story. Please check back for updates.