WASHINGTON, Dec 13 (Reuters) - The U.S. Centers for Disease Control and Prevention (CDC) on Monday advised Americans against travel to Italy, Greenland and Mauritius, citing COVID-19 concerns. The CDC now lists 84 destinations at "Level 4: Very High" classification, including nearly all of Europe. The United States imposed new rules, effective Dec. 6, requiring international air travelers arriving in the United States to obtain a negative COVID-19 test within one day of travel.