NEW YORK- Secretary of State Rex Tillerson says the Trump administration is considering closing down the U.S. Embassy in Havana following unexplained incidents that have affected the health of American diplomats.

Tillerson's comments are the strongest indication to date that the United States might mount a major diplomatic response, potentially jeopardizing the historic restart of relations between the U.S. and Cuba. The former foes reopened embassies in Washington and Havana in 2015 after a half-century of estrangement.

Tillerson says, "It's a very serious issue with respect to the harm that certain individuals have suffered."

Some of the 21 medically confirmed U.S. individuals affected - diplomats and their families - have permanent hearing loss or concussions, while others suffered nausea, headaches and ear-ringing. Some are struggling with concentration or common word recall.