The United States has ordered about 60 percent of its staff and their families at the U.S. Embassy in Havana to leave amid mysterious health attacks on American diplomats, senior State Department officials said Friday.

The exits will leave just a skeletal staff remaining at the embassy, which was only recently reopened after former President Obama restored diplomatic relations.

A senior State Department official said that the U.S. would stop issuing visas in Cuba indefinitely, and would stop official delegation travel to the country.

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The U.S. also issued a travel warning advising American citizens not to travel to Cuba, because of potential threats to their safety. The embassy will still provide emergency services to U.S. citizens in Cuba, but many routine services will be unavailable as a result of the drawdown.

Short-term travel to Cuba by U.S. officials will also be limited to those conducting the investigation into the attacks and those with business related to U.S. national security interests.

Secretary of State Rex Tillerson Rex Wayne TillersonGary Cohn: 'I haven't made up my mind' on vote for president in November Kushner says 'Alice in Wonderland' describes Trump presidency: Woodward book Conspicuous by their absence from the Republican Convention MORE reportedly made the decision to withdraw non-essential staff from the embassy after considering other options that included a total shutdown of the facility.

The Associated Press first reported the decision to withdraw non-emergency personnel from the embassy.

The cause of the symptoms remains unknown, and the U.S. investigators are looking into the matter.

A senior State Department official said that the drawdown will remain in effect until the Cuban government takes steps to secure the safety of Americans in the country.

The official said that the withdrawal of non-emergency personnel is not intended as a punitive measure, but is meant to ensure the safety of U.S. citizens in Cuba. The U.S. will maintain diplomatic relations with Havana, the official said.

Tillerson said in a statement that the U.S.’s relations with Cuba would continue “to be guided by the national security and foreign policy interests of the United States,” adding that investigators would continue to look into the matter. Cuba, he said, would also continue to conduct a separate investigation.

“The health, safety, and well-being of our Embassy community is our greatest concern,” he said. “We will continue to aggressively investigate these attacks until the matter is resolved.”

The Cuban government has denied responsibility for the so-called health attacks, and has offered the FBI unprecedented access to the country for its probe.

A State Department official said Friday that the U.S. has not ruled out the possibility that a third country could be responsible for the attacks.

Tillerson met this week with Cuban Foreign Minister Bruno Eduardo Rodriguez Parrilla. But that meeting reportedly did not offer reassurances that the Cuban government would take proper steps to ensure the safety of U.S. diplomats in the country.

The health attacks were revealed to the public in August, only when the State Department said that it had expelled two Cuban diplomats from the country’s embassy in Washington earlier this year.

Since then, the number of Americans who have experienced symptoms during their time in Havana has ticked upward.

So far, there have been 21 confirmed cases of the symptoms that range from hearing loss to balance problems to difficulty sleeping, a State Department official said Friday.

The attacks began in the fall of 2016 continued until March of this year. But the State Department revealed earlier this month that they occurred as recently as late August.

The attacks were initially thought to be caused by a covert sonic device. But U.S. investigators have yet to find any such device.