Sept. 13 (UPI) -- The U.S. Centers for Disease Control and Prevention on Monday added 10 destinations to its most severe level of travel warning.

The health agency designated Afghanistan, Albania, Belize, Grenada, Israel, Lithuania, Mauritius, Serbia, Slovenia and Saint Kitts and Nevis as "Level 4: Very High," urging Americans not to travel to these destinations unless they are fully vaccinated.

CDC criteria define "Level 4" countries as those reporting more than 500 cases per 100,000 residents in the past 28 days.

The agency warns that Americans who must travel to these destinations ensure that they are fully vaccinated but noted that "even fully vaccinated travelers may be at risk for getting and spreading COVID-19 variants."

Additionally, the CDC designated Australia, Brazil, Ethiopia and Romania as "Level 3: High."

Brazil was downgraded from Level 4, while the other destinations were upgraded from Level 2 "moderate."

Unvaccinated Americans are encouraged to specifically avoid travel to destinations designated at Level 3.

Broadly, the CDC advises that Americans avoid all international travel until they have been fully vaccinated.