US embassy staff have been ordered back to the States after a mysterious ailment afflicted diplomats in Havana.
Neither the Cubans nor the FBI have been able to infidelity the cause of the incidents, which they initially suspected to be futuristic "sonic attacks".
US President Donald Trump said "they did some very bad things" without saying who he meant by "they."
The US Embassy in Cuba will lose roughly 60 percent of its American staff and will stop processing visas for prospective Cuban travellers to the United States indefinitely.
Roughly 50 Americans had been working at the embassy.
Americans have been warned to stay away from the Caribbean country, a decision that could hit Cuba's economy that has expanded in recent years as the US relaxed restrictions.
The FBI and other agencies searched homes and hotels where incidents have occurred but found nothing to link to the ailments.
Secretary of State Rex Tillerson, who reviewed options for a response with president Trump, said: "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."
At least 21 diplomats and family members have been affected by the ailment whose symptoms include hearing loss, dizziness, headache, fatigue, cognitive issues and difficulty sleeping.
The Trump administration has pointedly not blamed Cuba for the attacks, and officials have spent weeks weighing how to minimise the risk for Americans in Cuba without harming relations with the country or falling into an adversary's trap.
Some investigators have suggested the attacks could have been committed by an outside power such as Russia or Venezuela to drive a wedge between the US and Cuba.