The Biden administration is investigating a recent spate of mysterious health incidents reported by American diplomats and other government employees in Vienna, say US officials.
Some of the symptoms are similar to those first reported by US diplomats and spies in Havana, Cuba, in 2016 and 2017, for which no definitive cause has yet been determined.
US officials said more than 20 new cases are being looked at by medical teams at the State Department and elsewhere, including the Pentagon and the CIA.
"In co-ordination with our partners across the US government, we are vigorously investigating reports of possible unexplained health incidents (UHI) among the US Embassy Vienna community," the State Department said.
"Any employees who reported a possible UHI received immediate and appropriate attention and care."
Some believe the unexplained injuries, which include brain damage, are the result of microwave or radio wave weapons, but despite years of study, there is no consensus as to what or who might be behind the incidents or whether they are actual attacks.
Employees in the Austrian capital have reported mysterious symptoms since President Joe Biden was inaugurated, according to the officials.
Vienna has for centuries been a centre for espionage and diplomacy and was a hub for clandestine spy-versus-spy activity during the Cold War.
The city is currently the site of indirect talks between Iran and the US over salvaging the nuclear deal that was negotiated there in 2015.
Those talks are on hiatus and it was not immediately clear if any members of the US negotiating team were among those suffering from injuries.
The problem has been labelled Havana Syndrome because the first cases affected personnel in 2016 at the US Embassy in Cuba.
People who are believed to have been affected have reported headaches, dizziness and symptoms consistent with a concussion, with some requiring months of medical treatment.
Some people recall hearing a loud noise before the sudden onset of symptoms.
Particularly alarming are revelations of at least two possible incidents in the Washington area, including one case in NOvember near the White House, in which an official reported dizziness.
Although some are convinced the injuries are the result of directed energy attacks, others believe the growing number of cases could be linked to "mass psychogenic illness", in which people learning of others with symptoms begin to feel sick themselves.
Cover image: View of the Vienna skyline, including the United Nations office complex (Picture: Jack Sullivan/Alamy Stock Photo).