The United States is considering closing down its recently re-opened embassy in Havana following a string of unexplained incidents harming the health of US diplomats in Cuba.

"We have it under evaluation," Secretary of State Rex Tillerson said on Sunday. "It's a very serious issue with respect to the harm that certain individuals have suffered. We've brought some of those people home. It's under review."

Of the 21 medically confirmed US victims - diplomats and their families - some have permanent hearing loss or concussions, while others suffered nausea, headaches and ringing in their ears. Some are struggling with concentration or common word recall, AP news agency has reported.

READ MORE: US expels Cuban diplomats over 'medical incidents'

Some victims felt vibrations or heard loud sounds mysteriously audible in only parts of rooms, leading investigators to consider a potential "sonic attack". Others heard nothing, but later developed symptoms.

Tillerson once called the events "health attacks", but the State Department has since used the term "incidents" while emphasising the US still does not know what has occurred.

Cuba has denied any involvement or responsibility but stressed it is eager to help the US resolve the matter.

Investigators have explored the possibility of sonic waves, an electromagnetic weapon, or an advanced spying operation gone awry. The US has not ruled out that a third country or a rogue faction of Cuba's security services might be involved.

Demoralising blow

A decision to shutter the embassy, even temporarily, would deal a demoralising blow to the delicate detente that former US President Barack Obama and Cuban leader Raul Castro announced in late 2014.

The next year embassies were re-opened, and restrictions on travel and commerce eased - signs of a warming relationship that displeased some hard-liners in Cuba's government.

President Donald Trump has reversed some of the changes but left many in place.

On Friday, five Republican senators wrote Tillerson urging him to not only shutter the embassy but also expel all Cuban diplomats from the US - a move with dramatic diplomatic implications.

"Cuba's neglect of its duty to protect our diplomats and their families cannot go unchallenged," said the legislators, who included Senator Tom Cotton of Arkansas, who led the effort, and Senator Marco Rubio of Florida, a prominent Cuban-American and critic of the US detente with Havana.