The United States in May expelled two Cuban diplomats based in Washington after unspecified "incidents" caused physical symptoms in Americans serving at the U.S. Embassy in Havana, the State Department spokeswoman said on Wednesday.
The exact nature of the incidents was unclear, but the Americans serving in Cuba returned to the United States "for medical reasons," State Department spokeswoman Heather Nauert told reporters.
The United States first learned of the incidents in late 2016, she said.
"We don't have any definitive answers about the source or the cause of what we consider to be incidents," Nauert said. "It's caused a variety of physical symptoms in these American citizens who work for the U.S. government. We take those incidents very seriously, and there is an investigation currently under way."
As a result, the United States on May 23 asked two Cuban officials in Washington to leave the country and they have done so, Nauert said.
The nature of the incidents was unclear, though Nauert said the Americans' ailments were not life-threatening.
"What this requires is providing medical examinations to these people," Nauert said. "Initially, when they'd started reporting what I will just call symptoms, it took time to figure out what it was, and this is still ongoing. So we're monitoring it."
Washington and Havana re-established diplomatic relations in 2015 after more than five decades of hostilities, re-opening embassies in each other's capitals and establishing a new chapter of engagement between the former Cold War foes.
President Donald Trump rolled back part of his predecessor Barack Obama's policy toward Cuba, but has left in place many of the changes, including the re-opened U.S. Embassy in Havana.
(Reporting by Mohammad Zargham and Yeganeh Torbati, editing by G Crosse)