A U.S. Navy service member on deployment in Japan died of unknown causes before testing positive for COVID-19, according to a base statement released on Tuesday.

The unnamed sailor was part of a cluster of 17 coronavirus cases detected at the Sasebo naval base in Nagasaki Prefecture since December 21. The service member, whose precise role will not be disclosed, "died of unknown causes and subsequently tested positive postmortem," said the statement on Facebook.

A spokesperson said there was no causal link between the death and the positive test.

Aki Nichols, Sasebo's public affairs officer, told American military newspaper Stars and Stripes that the cause of death was unknown. "It's circumstantial as far as we're concerned that the person was positive, we found that out after his death," he was quoted as saying.

Nichols told the paper he could not disclose any further personal details, including the service member's gender, military unit or position.

Among the remaining 16 active cases at Sasebo naval base, eight tested positive after exhibiting symptoms and eight during medical screening, its statement said. The individuals are in isolation.

Japan is home to the most forward-deployed American forces outside of the United States. Approximately 50,000 service personnel make up United States Forces Japan, the bulk of whom—just under 30,000—are stationed in Okinawa prefecture.

While other American bases across Japan are also experiencing winter COVID-19 spikes in the single digits or low 10s, U.S. Marine Corps base Camp Hansen, in Okinawa, is currently recording an alarming cluster, which grew to 277 active cases as of Tuesday.

On Monday, when the outbreak was 16 fewer, Japanese Chief Cabinet Secretary Hirokazu Matsuno said the U.S. had agreed to test its military personnel 72 hours before deployments. Camp Hansen has also reinstated a mask mandate for its troops.

While a number of Japanese employees of U.S. bases have tested positive for the highly transmissible Omicron variant, it remains unclear whether the new mutant has struck any service members in the country. The Japan Times has reported that the variant has been detected among Japanese communities in the prefectures of Tokyo, Kyoto, Osaka, Fukuoka, Aichi and Hiroshima.

Japan's COVID-19 cases have dropped significantly into the low-100s since peaking at more than 25,000 in a single day during the Tokyo Summer Games in August. On Monday, it reported 214 active cases, including 35 in capital Tokyo—the former epicenter.

Just over 78 percent of Japan's 125 million people were reported as fully vaccinated as of December 27.

U.S. Navy Member Dies At Japan Base
File: Sailors man the rails as the amphibious dock landing ship USS Germantown departs U.S. Fleet Activities Sasebo in Nagasaki, Japan, on September 15, 2021. U.S. naval base Sasebo reported on December 28 the death of a service member who later tested positive for COVID-19, but there has been no causal link established between the death and the disease.U.S. Navy/Mass Communication Specialist 1st Class Jeremy Graham