Austria and the US are investigating reports of diplomats in Vienna being sickened with mysterious health problems.
It is the latest in a series of unexplained episodes involving US diplomatic staff falling ill in Cuba, Russia and other countries.
Feared to be caused by deliberate attacks using focused energy beams, the incidents have left American staff with nausea, physical pain and even brain damage.
Fresh claims surfaced in the New Yorker this week that US diplomats and intelligence officers had reported similar problems in Vienna.
The report said that two dozen US operatives in Vienna had reported illnesses since President Joe Biden took office in January.
Vienna is home to a large corps of foreign diplomats and is hosting the talks between Iran and world powers aimed at restoring the 2015 nuclear deal.
Austria’s foreign ministry, which lists 158 US diplomats as being posted in Vienna, said it was working with US authorities on a joint investigation.
“We take these reports very seriously … the safety of the diplomats sent to Austria and their families is our top priority,” it said.
A US State Department representative said Washington was “vigorously investigating” the reports.
They said the employees who suffered unexplained health problems had received immediate attention and medical care.
There are suspicions that Russia is linked to the mystery ailments but the White House says it is still investigating what is causing the illnesses.
Scientists have theorised that the attacks are caused by pulsed microwave weapons.
US Secretary of State Antony Blinken said last month that the Biden administration was “committed to getting to the bottom of this.”
The subject was raised when Mr Biden met Russian President Vladimir Putin at a recent summit in Switzerland.
Major powers such as the US and Russia have many delegates in Vienna because it is home to several UN and other international organisations.
Both countries are part of efforts to return Washington to the nuclear deal with Iran which former president Donald Trump withdrew from in 2018.
Iran responded to the US departure by breaching the limits on its nuclear activity that it agreed to under the deal, known as the Joint Comprehensive Plan of Action (JCPOA).
US negotiator Robert Malley said on Saturday that the talks were at an impasse, which he blamed on Iran.
“We stand ready to return to Vienna to complete work on a mutual return to the JCPOA once Iran has made the necessary decisions,” he said.