The United States on Friday abruptly warned Americans not to visit Cuba and ordered more than half its Havana embassy personnel to leave the island in a dramatic response to mysterious recent "specific attacks" harming the health of US diplomats.
The actions dealt a blow to already-delicate ties between the US and Cuba, long-time adversaries who only recently began putting their hostility behind them.
The embassy in Havana will lose roughly 60% of its US staff, and will stop processing visas in Cuba indefinitely, officials said.
About 50 Americans are currently working at the embassy in Havana.
"Until the government of Cuba can ensure the safety of our diplomats in Cuba, our embassy will be reduced to emergency personnel in order to minimise the number of diplomats at risk of exposure to harm," Secretary of State Rex Tillerson said.
He and President Donald Trump reviewed the options in a White House meeting this week, officials said.
Mr Trump said there was a "big problem" in Cuba, as he responded to a shouted question as he left the White House,.
He said "they did some very bad things" but did not specify who he believed was responsible.
In Friday's travel warning, the State Department said some of the unexplained physical effects have occurred in Cuban hotels, and that while American tourists are not known to have been hurt they could be exposed if they travel to Cuba.
Tourism is a critical component of Cuba's economy that has grown in recent years as the US has relaxed restrictions.
"Over the past several months, numerous US Embassy Havana employees have been targeted in specific attacks," the department said in the warning.
"These employees have suffered significant injuries as a consequence of these attacks.
"Affected individuals have exhibited a range of physical symptoms including ear complaints and hearing loss, dizziness, headache, fatigue, cognitive issues and difficulty sleeping.
"Because our personnel's safety is at risk, and we are unable to identify the source of the attacks, we believe US citizens may also be at risk and warn them not to travel to Cuba," it said.
"Attacks have occurred in US diplomatic residences and hotels frequented by US citizens."
For now, the United States is not ordering any Cuban diplomats to leave Washington, a move the administration had considered, officials said.
Several US politicians have called on the administration to expel all Cuban diplomats.
In May, Washington asked two to leave, but emphasised it was to protest at Havana's failure to protect diplomats on its soil, not an accusation of blame.
Almost a year after diplomats began describing unexplained health problems, US investigators still do not know what or who is behind the attacks, which have harmed at least 21 diplomats and their families.
Until now, the State Department has called them "incidents" and generally avoided deeming them attacks.
Mr Tillerson made the decision to draw down the embassy after considering other options that included a full embassy shutdown and less significant personnel reductions.
To medical investigators' dismay, the symptoms in the attacks vary widely from person to person.
In addition to hearing loss and concussions, some experienced nausea, headaches and ear-ringing, and The Associated Press has reported some now suffer from problems with concentration and common word recall.
Though officials initially suspected some futuristic "sonic attack", the picture has grown muddier.
The FBI and other agencies that searched homes and hotels where incidents occurred found no devices.
Clues about the circumstances of the incidents seem to make any explanation scientifically implausible.
Some US diplomats reported hearing loud noises or feeling vibrations when the incidents occurred, but others heard and felt nothing yet reported symptoms later.
In some cases, the effects were narrowly confined, with victims able to walk "in" and "out" of blaring noises audible in only certain rooms or parts of rooms, the AP has reported
The United States notified Cuba early Friday via its embassy in Washington.
The moves deliver a significant setback to the delicate reconciliation between the US and Cuba, two countries that endured a half-century estrangement despite their locations only 90 miles apart.
In 2015, President Barack Obama and Cuban President Raul Castro restored diplomatic ties, embassies re-opened, and travel and commerce restrictions were eased.
Mr Trump has reversed some changes, but has broadly left the rapprochement in place.
The Trump administration has pointedly not blamed Cuba for perpetrating the attacks.
Though Cuba implored the United States not to react hastily to the reports of health attacks, it appeared that last-minute lobbying was unsuccessful in the highest-level diplomatic contacts between the countries since the start of Mr Trump's presidency in January.