The State Department told Americans not to travel abroad at all, the strongest U.S. alert yet as the novel coronavirus continued its steady march across the globe.

The department on Thursday issued a Level 4 advisory for travel abroad – "do not travel" – only four days after it issued a Level 3 advisory that urged Americans to "reconsider travel."

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"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," the advisory said. "U.S. citizens who live abroad should avoid all international travel."

© Luca Bruno, AP A man wearing a mask rides a scooter in Milan, Italy, March 11, 2020. Italy is mulling even tighter restrictions on daily life and has announced billions in financial relief to cushion economic shocks from the coronavirus.

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The advisory came as the number of cases have multiplied: More than 11,000 in the United States out of 236,000 worldwide. The global death toll also neared 10,000 on Thursday, including 157 in the United States.

The advisory is not a mandate; Americans can still leave the U.S.

But they do so at their own risk. Hundreds of Americans have been stranded overseas amid a global freeze on international travel and mass quarantines sparked by the coronavirus pandemic.

Besieged by pleas for help, the State Department and its embassies around the world have offered little to no assistance, these stranded travelers say.

"I can't help but feel we are abandoned," Chris Pierce, who is stuck in the Philippines with his wife and 9-year-old son, told USA TODAY.

"I'm a realistic person, I do realize they are probably overwhelmed," Pierce said. "But at the same time, all U.S. citizens should be a priority."

Sen. Robert Menendez, the top Democrat on the Senate Foreign Relations Committee, urged the State Department to begin chartering commercial airplanes to bring stranded Americans home. He said the Trump administration should use an existing civil airfleet program for such evacuations.

“No American citizen should be abandoned overseas as we confront this unprecedented pandemic simply because of a failure of government to provide them the support that they need,” Menendez said Thursday. “If there ever was a need to increase our nation’s aircraft capability during a national crisis, this is it.”

The New Jersey Democrat said the Pentagon should also explore the use of military aircraft to repatriate Americans.

A State Department official, who was not authorized to speak on the record, said the agency could not provide an estimate of how many Americans are trying to get back to the U.S. from overseas amid the COVID-19 outbreak.

The official said the State Department is considering all options to assist U.S. citizens, and that it is continuously assessing travel conditions in all areas affected by COVID-19.

“This is an unprecedented action,” Matthew Schmidt, an associate professor of national security and political science at the University of New Haven, said of the new Level 4 travel advisory.

Not only have Americans been told to cancel travel overseas, but those already abroad “have been temporarily abandoned by their government,” he said. “Such is the crisis we face that these measures are necessary.”

In its advisory Thursday, the State Department noted that foreign governments are closing their borders, implementing mandatory quarantines, and barring entry and exit with little advance notice.

"Airlines have cancelled many international flights and several cruise operators have suspended operations or cancelled trips," the advisory said. "If you choose to travel internationally, your travel plans may be severely disrupted, and you may be forced to remain outside of the United States for an indefinite timeframe."

Contributing: Morgan Hines, USA TODAY

This article originally appeared on USA TODAY: Coronavirus: State Department tells Americans: 'Do not travel' abroad, come home if overseas