Japan has confirmed its first case of a Japanese national infected with the Omicron variant of the coronavirus, marking the third case in the country, government sources said Monday.

The man in his 30s, who had traveled to Italy, was found to be infected with the heavily mutated variant after entering Japan through Tokyo's Haneda airport on Wednesday, according to the health ministry.

He is fully vaccinated after receiving two doses of Moderna Inc.'s COVID-19 vaccine in July and August, the sources said.

Prime Minister Fumio Kishida pledged Monday to "concentrate our limited medical resources on the Omicron strain and take swift measures to protect the lives of the people."

According to the Ministry of Health, Labor and Welfare, all of the other 41 passengers on the same flight as the man are being treated as close contacts. One has tested positive for the coronavirus but not the Omicron variant.

Japan confirmed its first domestic case of the Omicron variant on Nov. 30 -- a Namibian diplomat in his 30s. It confirmed a second unrelated case, a man in his 20s who traveled from Peru, the following day.

Both men tested positive on arrival at Narita airport near Tokyo.

The Omicron variant has already been detected in dozens of countries including the United States, Australia and parts of Europe since being first reported by South Africa last month.

Scientists are working to discover whether the highly mutated strain is more transmissible than the existing Delta variant of the coronavirus or able to evade immunity obtained from vaccines or previous infections.

Related coverage:

Japan PM Kishida vows to "prepare for worst" amid Omicron variant fears

Japan to bring forward COVID booster shots "wherever possible"

Japan expands tougher quarantine rules to India, Greece, Romania