The Trump administration is expelling 15 diplomats from Cuba's embassy in Washington following last week's US move to pull more than half of its own diplomats out of Havana.
US Secretary of State Rex Tillerson said on Tuesday the latest decision was made due to Cuba's "failure to take appropriate steps" to protect American personnel in Cuba who have been targeted in mysterious "attacks" on their health.
The steps being taken by President Donald Trump's administration mark a further blow to his predecessor Barack Obama's policy of rapprochement between Washington and Havana, former Cold War foes.
A State Department official said the number of expulsions was selected to make sure the US and Cuban embassies would have "equitable staffing levels" while investigations continue into the unexplained "health attacks".
The US decision to expel a large portion of Cuban staff at the embassy was communicated to Cuban Ambassador Jose Ramon Cabanas on Tuesday, and the diplomats were given seven days to leave the United States, the official said, speaking on condition of anonymity.
The move follows an announcement on Friday that the United States was sharply reducing its diplomatic presence in Cuba as it warned US citizens not to visit because of attacks that have caused hearing loss, dizziness and fatigue in US embassy personnel.
"Until the Government of Cuba can ensure the safety of our diplomats in Cuba, our embassy will be reduced to emergency personnel to minimise the number of diplomats at risk of exposure to harm," Mr Tillerson said in a statement.
"We continue to maintain diplomatic relations with Cuba, and will continue to cooperate with Cuba as we pursue the investigation into these attacks," he added.
The number of American diplomats suffering symptoms has increased to 22, the State Department official said.
The official maintained that despite the US moves, Washington was not assigning "culpability" to Cuba's Communist government.