The U.S. has raised its travel advisory for South Korea by a notch to the highest level due to COVID-19 amid the rapid spread of the omicron variant.

The Centers for Disease Control and Prevention(CDC) designated South Korea as a Level Four destination in its four-tier travel alert system due to "very high" levels of the virus.

Per the CDC’s metrics, countries are considered to have "very high" COVID-19 levels when they report more than 500 new cases per 100-thousand people over the past 28 days.

Azerbaijan, Belarus, Comoros, Saint Pierre and Miquelon and French Polynesia were also moved to the highest category.

In the update posted on its website, the CDC advised American citizens to avoid travel to these destinations and to get vaccinated if they must travel.

The latest update increased the number of destinations classified as Level Four to 137.