WASHINGTON — The State Department announced on Thursday that Americans should not travel outside the country, and that citizens abroad should either return home or stay in place as the coronavirus pandemic grows.

The department raised its global health advisory to Level 4, or “do not travel,” which is the highest warning, usually reserved for nations that are war zones or facing serious disruptions such as political unrest or natural disasters.

The announcement is a recommendation, not a requirement. Millions of Americans are still overseas, and many are likely to opt to remain in place.

“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 time frame,” the advisory said.