WASHINGTON (AP) — The Trump administration has upgraded its already dire warning to Americans against all international travel as the coronavirus outbreak spreads.

The State Department on Thursday issued a new alert urging Americans not to travel abroad under any circumstances and to return home if they are already abroad unless they plan to remain overseas.

"The Department of State advises U.S. citizens to avoid all international travel due to the global impact of COVID-19," it said in the new advice. "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."

Until the upgrade, the department's advice to U.S. citizens was to "reconsider" all international travel under what is known as a "level three" alert. The global "level four" warning was unprecedented as such alerts are generally reserved for specific countries embroiled in conflict, natural disasters or where Americans face specific risks.

However, the upgrade will likely have little practical effect because it is not mandatory and there are now limited transportation options for international travel. The only way to ban Americans from going abroad would be to invalidate the use of U.S. passports for such travel, a bar that is currently in place only for North Korea.

In addition, the main impact of State Department travel alerts is to cause insurance companies to increase premiums or cancel travel policies for group and individual tours, many of which had been scrapped even before the alert was raised to level three earlier this week.

The department has already advised Americans that many U.S. embassies and consulates abroad are operating with reduced staff and hours due to the COVID-19 outbreak and that services for Americans in need of assistance are limited.