Japan reported four new cases of the omicron variant of the coronavirus at airport screening sites, the health ministry said Monday, bringing the total number of cases detected in the nation to 17.

According to the health ministry, the patients, both male and female in their 20s through their 40s, arrived in Japan between Dec. 6 and Thursday aboard flights from Nigeria, Kenya and the United States.

Japan closed its borders earlier this month to all arrivals except Japanese citizens and foreign residents amid the spread of the variant, which is believed to be highly transmissible.