The US is withdrawing more than half of its staff at its embassy in Cuba in response to mysterious attacks which left its diplomats unwell.
Washington is also warning Americans not to visit the country because some attacks occurred in hotels.
At least 21 staff reported health problems ranging from mild brain trauma and deafness to dizziness and nausea.
Earlier reports suggested that sonic attacks were to blame. Cuba denies any involvement in them.
At least two Canadians were also affected.
US Secretary of State Rex Tillerson said America would maintain diplomatic ties with Cuba and the two countries would continue to co-operate in investigating the attacks.
The US is ordering all non-essential staff in the embassy in Havana to leave, along with all family members. Only "emergency personnel" will remain.
The US has suspended visa processing in Cuba indefinitely.
"Until the government of Cuba can ensure safety of our people, we will be reduced to emergency personnel," a US state department official said.
"At least 21 employees have been targeted in specific attacks," the official said.
Despite an investigation involving the FBI, the Royal Canadian Mounted Police and the Cuban authorities, there is still no full explanation as to the cause of the incidents since late 2016.
"We don't know the means, the methods or how the attacks are being carried out," the official said on Friday.
But earlier reports suggested they were sonic attacks in which staff in Havana were targeted by audio waves, leaving several with chronic hearing issues.
Using an inaudible sound device for a stealth attack "is quite plausible from a technical point of view", Denis Bedat, a specialist in bio-electromagnetics, told AFP news agency this week.
"Ultrasonic waves, beyond the acoustic capacity of humans, can be broadcast with an amplifier, and the device does not need to be large, or used inside or outside a house."
He gave as an example the Active Denial System (ADS), an anti-riot gun used by US police forces that emits electromagnetic waves which produce a sudden unbearable burning sensation.
The US has not blamed Cuba for the suspected attacks.
Both the US and Cuban governments "have not yet identified the guilty party", the state department official said.
"We have not ruled out the possibility of a third country as a part of the investigation but that investigation continues," the official added.
President Raul Castro had reportedly given his personal assurance to the then-US charge d'affaires in Havana that Cuba was not behind the attacks.
The US reopened its embassy in Havana in 2015 following decades of frozen relations.
In 2016 President Barack Obama became the first sitting US president to visit Cuba since Calvin Coolidge in 1928.
In June President Donald Trump announced a partial roll-back of Mr Obama's Cuba policies but said he would not close the US embassy in Havana.