The United States, in response to the coronavirus pandemic, has raised its travel advisory to a "level four", its highest level, instructing citizens abroad to return home or shelter in place indefinitely.

Citizens of the US are also instructed not to travel abroad, according to the advisory, which was posted on the US Department of State's website on Thursday.

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"The Department of State advises US citizens to avoid all international travel due to the global impact of COVID-19," the advisory says. "In countries where commercial departure options remain available, US 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. US citizens who live abroad should avoid all international travel."

The level of alert being applied globally is unprecedented, according to the Associated Press news agency.

The measure comes as thousands of Americans have been stranded abroad as a result of cancelled flights and closed borders.

The State Department has warned that US embassies are currently working with reduced staffing around the world.

On Wednesday, the US suspended all routine visa services in most countries worldwide due to the coronavirus outbreak, a spokeswoman for the State Department said.

The virus has infected over 207,000 worldwide, and the disease it causes, COVID-19, has killed over 8,600 since first appearing the Chinese city of Wuhan in December.

Level three to level four

Under the previous US travel advisory, a "level three" alert, citizens were instructed to "reconsider" all international travel. The "level four" alert is generally reserved for specific countries embroiled in conflict, natural disasters or where Americans face specific risks.

Travel Advisory: Level 4 - The Department of State advises U.S. citizens to avoid all international travel due to the global impact of #COVID19. In countries where commercial departure options remain available, U.S. citizens who live in the US should arrange for immediate return. pic.twitter.com/MydSzFffYd — Travel - State Dept (@TravelGov) March 19, 2020

However, the alert is not mandatory, so it is unclear how much of a practical effect it will have. The only way to ban Americans from going abroad would be to invalidate the use of US passports for such travel, a measure that is currently in place only for North Korea.

State Department travel alerts mainly affect insurance companies, causing them 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.