President Trump’s administration Thursday afternoon upgraded its warning to Americans against all international travel as the coronavirus pandemic continues to spread.

The State Department has issued a new alert which urges Americans not to travel abroad under any circumstances and to return home if they are already abroad, unless they are planning to remain overseas for the foreseeable future.

“The Department of State advises U.S. citizens to avoid all international travel due to the global impact of COVID-19,” the alert reads. “In countries where commercial departure options remain available, U.S. citizens who live in the United States should arrange for immediate return to the United States, unless they are prepared to remain abroad for an indefinite period. U.S. citizens who live abroad should avoid all international travel.”

Before the upgraded warning, the department’s advice to U.S. citizens was to “reconsider” all international travel under a “level three” alert. The new, global “level four” warning is typically reserved for specific countries embroiled in conflict, natural disasters or where Americans face specific risks.

However, the upgrade is not mandatory, and there are currently limited transportation options for international travel. The only way to ban Americans from traveling abroad would be to invalidate the use of U.S. passports, a ban that is currently in place only for North Korea.

In addition, the main impact of such travel alerts is to cause insurance companies to increase customers’ premiums or to cancel travel policies for group and individual tours. Many of those had already been cancelled before the alert was raised to level three earlier this week.

On a related note, the State Department has already advised Americans that many U.S. embassies and consulates around the world are operating with reduced staff and hours due to the COVID-19 outbreak, and that services for Americans who are in need of assistance are limited.