The U.S. State Department is warning Americans against traveling on cruise ships due to concerns over coronavirus.

In a tweet, the State Department said: “U.S. citizens, especially with underlying conditions, should not travel by cruise ship. CDC notes increased risk of #COVID19 on cruises. Many countries have implemented screening procedures, denied port entry rights to ships and prevented disembarking.

“In some cases, local authorities have permitted disembarkation but subjected passengers to local quarantine procedures. While the U.S. government has evacuated some cruise ship passengers in recent weeks, repatriation flights should not be relied upon as an option for U.S. citizens under the potential risk of quarantine by local authorities.”

U.S. citizens, especially with underlying conditions, should not travel by cruise ship. #CDC notes increased risk of #COVID19 on cruises. Many countries have implemented screening procedures, denied port entry rights to ships and prevented disembarking. https://t.co/jh93gZTkpC pic.twitter.com/jI6S0UceVg — Travel - State Dept (@TravelGov) March 8, 2020

The CDC is also recommending older adults and travelers with underlying health issues avoid crowded places and non-essential travel such as long plane trips.

Cruise ships have turned into an area of focus for those working to fight the spread of COVID-19, the illness caused by coronavirus. The Grand Princess cruise ship is currently off the coast of California and is expected to dock Monday after 21 people on board tested positive for coronavirus.

All passengers will undergo a 14-day quarantine once they leave the ship.

According to USA Today, some 962 of the ship’s 2,421 passengers are from California and will be quarantined at Travis Air Force Base and Miramar Naval Air Station in California. U.S. citizens from other states will be taken to Joint Base San Antonio Lackland in Texas or Dobbins Air Force Base in Georgia. Passengers from other countries will be flown out of the U.S. on charter flights.

Another Princess Cruise line ship, the Regal Princess, is being held off the coast of Florida awaiting results from tests on two crewmembers. The crew members in question had transferred from the Grand Princess cruise ship in California.