The Trump administration is preparing for a major drawdown of personnel and family from the U.S. Embassy in Havana amid growing concerns over mysterious health attacks targeting American diplomats.

CBS News reported the development on Thursday, noting that an internal memo had been sent to 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 advising that the U.S. withdraw nonessential employees in Cuba.

Tillerson met with Cuban Foreign Secretary Bruno Eduardo Rodriguez Parrilla this week, but the meeting did not offer the secretary any reassurance that Havana was taking steps to protect U.S. diplomats in the country, according to CBS.

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Last fall, U.S. diplomats in Havana began reporting mysterious symptoms, including hearing loss, headaches and balance issues.

The attacks appeared to stop in March, but the State Department revealed earlier this month that reports of a new attack had been received as recently as August. So far, at least 21 cases have been confirmed.

U.S. officials initially believed the symptoms were being caused by a covert sonic device. But U.S. investigators looking into the matter have not found any such device, and the cause of the symptoms remains unknown.

The health attacks only became known to the public in August, when the State Department revealed that it had expelled two Cuban diplomats from the country's embassy in Washington in response to the matter.

The Cuban government has forcefully denied that it is behind the attacks, and has allowed the FBI unprecedented access in the country to investigate.

McClatchy reported on Tuesday that the Trump administration does not believe the Cuban government is responsible for the attacks on U.S. diplomats. A withdrawal of U.S. personnel from the embassy would not be intended as a punitive measure, but rather to ensure the safety of diplomats and their families.

In an appearance on CBS's "Face the Nation" earlier this month, Tillerson suggested that the U.S. could close the Havana embassy, which reopened in 2015 amid efforts by the Obama administration to restore diplomatic relations with Cuba.

The American Foreign Service Association (AFSA), the union representing the U.S. Foreign Service, came out against the proposal to withdraw personnel from the embassy in Havana, telling BuzzFeed News that doing so would ultimately disrupt the diplomatic mission in there.

“We have a mission to do,” Barbara Stephenson, the union's president, said. “AFSA’s view is that American diplomats need to remain on the field and in the game.”