The US State Department on Thursday warned American citizens not to travel abroad due to the coronavirus pandemic, issuing its highest level travel alert.

The Level 4: Do Not Travel advisory “advises U.S. citizens to avoid all international travel due to the global impact of COVID-19.”

“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 alert said.

The State Department alert advised US citizens living abroad to “avoid all international travel.”

“Many countries are experiencing COVID-19 outbreaks and implementing travel restrictions and mandatory quarantines, closing borders, and prohibiting non-citizens from entry with little advance notice,” it said.

“Airlines have cancelled many international flights and several cruise operators have suspended operations or cancelled trips. 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,” it said.

Four sources told CNN earlier on Thursday that Secretary of State Mike Pompeo had approved the Level 4 advisory.

Last week the advisory was raised globally to Level 3: Reconsider Travel “due to the global impact of COVID-19.”

Asked about the advisory, President Donald Trump said that he couldn’t “say right now” but noted he would be “speaking with the State Department later.”

Politico was first to report the decision to raise the advisory level.

Another source told CNN that the move would be aimed at curtailing Americans’ travel abroad and to get those who are traveling to come home.

However, hundreds of Americans are already abroad, including in countries which have restricted travel and closed their borders in an effort to contain the outbreak. Many of those who are stranded say they are struggling to get help from the State Department.

A group of senators wrote to Pompeo Wednesday to demand answers, saying they are “particularly concerned about an increasing number of reports that Americans and their family members have been unable to leave areas affected by COVID-19 and return home.”

Pompeo said Wednesday the department is “doing everything we can to protect American citizens all across the world.”

“We know of students that are in Peru, some other travelers that are there as well. There are other countries, too, where those countries have shut down their airports,” he said in an interview with Sean Hannity. “We are working to try and solve problems for each of those American citizens. We just learned about them over the last couple days; it’ll take us some amount of time. But know that President Trump has made clear that we’re going to do everything we can to get every American home safely.”

Prior to the coronavirus pandemic, the Level 4 travel advisory had been used sparingly for only a handful of countries, including Syria, Iran, Yemen and North Korea. However, as the virus spread around the globe, the department raised the advisory to the top level for a number of additional countries, including China and Mongolia, as well as regions within certain countries.

Last weekend, the State Department “authorized the departure from any diplomatic or consular post in the world of US personnel and family members who have been medically determined to be at higher risk of a poor outcome if exposed to COVID-19.