The U.S. State Department is warning U.S. citizens against all international travel.

The department issued on Thursday a level four “Do Not Travel" advisory, its strongest warning, cautioning all U.S. citizens to avoid traveling overseas due to the global COVID-19 pandemic.

It also urged U.S. citizens already overseas to make immediate preparations to return home unless they are prepared to remain overseas indefinitely. It advised U.S. citizens living overseas to avoid international travel as well.

The move is seen as a strong measure to stop the spread of COVID-19.

The Associated Press reported that the move is unprecedented as level four advisories have till now been issued only for specific countries suffering from conflict or natural disasters or where U.S. citizens faced threats, not the entire world. Prior to this, level four advisories were in place for China, Iran, Mongolia, the South Korean city of Daegu and the Italian regions of Lombardy and Veneto.