The United States is withdrawing 60 percent of its diplomatic staff and all family members from its embassy in Cuba after 21 diplomats suffered mysterious health attacks in recent months, US officials said.

Washington is investigating the cause of the unexplained attacks and has not yet identified the responsible party, but officials on Friday stressed the Cuban government was accountable for protecting foreign diplomats on its territory.

The Cuban government has denied any role and has also launched an investigation. Last week, Havana said it found no evidence to support US claims that several American diplomats were harmed in what US Secretary of State Rex Tillerson called surreptitious "health attacks".

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"Until the government of Cuba can ensure the safety of US government personnel in Cuba, our embassy will be reduced to emergency personnel so as to minimize the number of US government personnel at risk of exposure," a US official told reporters on Friday, speaking on the condition of anonymity to discuss the decision.

In response, Cuba's government criticised the US decision as "hasty" and warned the move would affect ties.

"We consider the decision announced by the US government today, through the State Department, to be hasty, and it will affect bilateral relations," said Josefina Vidal, head of North American affairs at Cuba's foreign ministry.

'Bizarre international mystery'

The US diplomats experienced unexplained health problems from attacks that occurred as recently as August, including ear complaints, hearing loss, dizziness, tinnitus, headache, fatigue, cognitive issues and difficulty sleeping, officials said.

The union that represents US diplomats has said the attacks have resulted in "mild traumatic brain injury" and "permanent hearing loss".

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. And clues about the circumstances of the incidents seem to make any explanation scientifically implausible.

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

Al Jazeera's Patty Culhane, reporting from Washington, DC, described the case as a "bizarre international mystery".

"The US is taking these steps and you might ask, "Do they have some evidence? Was Cuba involved?' Well, the answer is 'no'," she said.

"Senior US State Department officials say the investigation is ongoing and that they still have absolutely no idea what harmed their diplomats.

"They say they are taking this step because they want to reduce the risk to American diplomats, they say that this isn't meant to punish Cuba ... [but] a precautionary step."

Travel warning

The US embassy will continue to remain open to fulfil basic diplomatic functions and provide emergency services, but routine visa processing will be suspended, according to US officials.

Washington would no longer send official delegations to Cuba or conduct bilateral meetings in Cuba, but would hold any talks in the US.

The US is also issuing a travel warning, telling all US citizens to avoid travel to Cuba. Because some of the attacks occurred at a hotel, the US government was unable to guarantee the safety of US travellers, even though only diplomatic staff have been targeted thus far, the officials said.

Later on Friday, Canada said it has no plans to change its travel advice for Cuba or to remove any of its diplomats from its embassy there, though some staff had experienced some "unusual symptoms in March 2017, including earaches, dizziness and headaches".

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Tillerson met his Cuban counterpart, Bruno Rodriguez Parrilla, in Washington on Tuesday amid heightened tensions over the unexplained attacks.

Barack Obama, the former US president, reopened diplomatic relations with Cuba after more than 50 years and the embassy resumed operations in 2015.

US President Donald Trump has taken steps to roll back some of Obama's reforms towards Cuba but has maintained the embassy in Havana.

Trump has said his administration would not lift sanctions on Cuba until it makes fundamental reforms and threatened to take further action against Venezuela, a recent target of US sanctions, if Cuba did not change course.