Secretary of State Rex Tillerson said Sunday that the Trump administration is considering whether to close the U.S. Embassy in Havana following a string of unexplained incidents that have damaged the health of American diplomats.

“We have it under evaluation,” Tillerson told CBS News’ “Face the Nation.” “It's a very serious issue.”

At least 21 Americans have been confirmed to have suffered some kind of medical harm in Havana. Tillerson also confirmed the State Department has brought home some of the people affected.

He has previously called the episodes "health attacks." But the State Department now refers to them as "incidents."

Their cause and culprits have yet to be determined. However, U.S. officials said the victims suffered from hearing loss and, in some cases, mild brain damage, possibly from sound waves. Cuban President Raul Castro has claimed his government had nothing to do with it.

Tillerson spoke amid calls from some U.S. senators to shutter the embassy in Cuba’s capital.

Last week, State Department spokeswoman Heather Nauert acknowledged the administration was at least considering pulling some staff from the embassy.

Nauert said it “obviously” was a dangerous situation, adding, “We are tremendously concerned about that. … Our folks can come back to the United States if they wish to do so. It shows the bravery, the hard work and the dedication of Americans, whether they are serving in Cuba or whether they are serving anywhere across the world. … I want to recognize them and let them know that we care, we certainly have not forgotten about them, and that this investigation is aggressive.”

Fox News’ Rich Edson and The Associated Press contributed to this report.