The Trump administration is preparing to ask Cuba to withdraw 60 per cent of its diplomats from Washington, US officials said on Monday, in response to last week’s US move to cut its own embassy staff in Havana by a similar amount.

The US request marks yet another major setback for relations between the United States and Cuba, two countries that only recently renewed diplomatic relations after a half-century of hostility. It comes as the US seeks to protect its own diplomats from unexplained attacks that have harmed at least 21 Americans in Havana with ailments that affected their hearing, cognition, balance and vision.

The State Department is expected to announce the decision on Tuesday, officials said, though they cautioned no decision was formalised until publicly announced. The officials weren’t authorised to discuss the plan publicly and requested anonymity.

Secretary of State Rex Tillerson discussed the plan Monday with President Donald Trump, one of the officials said.

Cuba has denied involvement in the attacks. Though Havana is likely to view the move as unwarranted retaliation, US officials said the goal wasn’t to punish the communist-run island, but to ensure both countries have a similar number of diplomats in each other’s capitals. The United States will formally ask Cuba to pull the diplomats, but won’t expel them forcefully unless Havana refuses, the officials said.

Tensions between the two neighbours have been escalating amid serious US concern about the unexplained attacks on Americans in Havana.

On Monday, The Associated Press reported that US spies were among the first and most severely affected victims. Though bona fide diplomats have also been affected, it wasn’t until US spies, working out of the embassy under diplomatic cover, reported hearing bizarre sounds and experiencing even stranger physical effects that the United States realised something was wrong, several individuals familiar with the situation said.

The mysterious “health attacks” started within days of President Donald Trump’s election in November, the AP has reported. But it wasn’t until Friday that the United States ordered more than half its embassy staff to return home.

Delivering a one-two punch to US-Cuba relations, the US last week also delivered an ominous warning to Americans to stay away from Cuba, a move that could have profound implications for the island’s travel industry. The US said that since some workers had been attacked in Havana hotels, it couldn’t assure Americans who visit Cuba that they wouldn’t suffer attacks if they stay in hotels there.

Cuba had blasted the American move as “hasty” and lamented that it was being taken without conclusive investigation results. But several US lawmakers had said that move by Washington didn’t go far enough, because President Raul Castro’s government was being permitted to keep all of its diplomats in the US Senator Marco Rubio, Republican-Florida, had called the one-sided action “an insult” in an AP interview.

There was no immediate reaction from Cuba’s Embassy in Washington late Monday after word emerged that the US planned to ask Cuban diplomats to leave. Yet the move will bring the two countries closer to the chilly state of relations they endured for decades until 2015, when they restored formal ties and re-opened embassies in Havana and Washington.

The US previously had roughly 50 American workers at its embassy in Havana, so the 60 percent reduction will bring the figure down to roughly 20. It wasn’t immediately clear late Monday how Cuban diplomats will have to leave Washington to bring the two countries’ rosters to parity.

In Friday’s travel warning urging Americans not to visit Cuba, the State Department confirmed earlier reporting by the AP that US personnel first encountered unexplained physical effects in Cuban hotels. While American tourists aren’t known to have been hurt, the US said they could be exposed if they travel to the island.

“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,” the warning said.

At least 21 diplomats and family members have been affected. The department said symptoms include hearing loss, dizziness, headache, fatigue, cognitive issues and difficulty sleeping. Until Friday, the US had generally referred to “incidents.” Tillerson’s statement ended that practice, mentioning “attacks” seven times; the travel alert used the word five times.

Still, the administration has pointedly not blamed Cuba for perpetrating the attacks, and officials have spent weeks weighing how to minimise the risk for Americans in Cuba without unnecessarily harming relations or falling into an adversary’s trap.

In 2015, President Barack Obama and Cuban President Raul Castro restored diplomatic ties, ordered embassies re-opened and eased travel and commerce restrictions. Trump has reversed some changes but has broadly left the rapprochement in place.

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To medical investigators’ dismay, symptoms have varied widely. In addition to hearing loss and concussions, some people have experienced nausea, headaches and ear-ringing. The Associated Press has reported some now suffer from problems with concentration and common word recall.

Though the incidents stopped for a time, they recurred as recently as late August.