The US government is investigating a series of health incidents in the Austrian capital Vienna involving its diplomats and other administration staff.
More than 20 officials have reported symptoms similar to Havana Syndrome - a mystery brain illness - since President Joe Biden took office in January.
The syndrome is unexplained, but US scientists say it is most probably caused by directed microwave radiation.
It was first found in Cuba in 2016-17.
US and Canadian diplomats in Havana complained of symptoms ranging from dizziness, loss of balance, hearing loss and anxiety to something they described as "cognitive fog".
The US accused Cuba of carrying out "sonic attacks", which it strongly denied, and the incident led to increased tension between the two nations.
A 2019 US academic study found "brain abnormalities" in the diplomats who had fallen ill, but Cuba dismissed the report.
The Vienna cases first came to light in the New Yorker magazine on Friday and were later confirmed by the US State Department, which said it was "vigorously investigating".
Reuters quoted an Austrian foreign ministry statement saying it was "working with the US authorities on jointly getting to the bottom of this".
Vienna has long been a centre for diplomatic activity and has had a reputation as a hotspot for espionage, particularly during the Cold War.
Countries like the US have a large diplomatic presence there.
The city is currently hosting indirect talks between Iran and the US over attempts to resurrect the 2015 Iran nuclear deal.
Cases of the condition have been reported elsewhere in the world, but US officials say the numbers in Vienna are greater than in any other city apart from Havana.
In June, US Secretary of State Antony Blinken announced a wide-ranging review into the causes of the illness.