The US government is reported to have issued a new warning over "very real" Havana Syndrome, urging diplomats and intelligence officers to immediately flee the area if they begin experiencing symptoms.
The White House sent a memo to the Pentagon last week that "anomalous health incidents" - also known as "Havana Syndrome" because it was first detected in Cuba - pose a real and serious danger.
Those experiencing a sudden and acute onset of pressure, sound or heat in the head should report them as soon as possible.
Havana Syndrome emerged in Cuba in 2016 when two-dozen CIA officers reported suffering from a "buzzing" and intense pressure in their skull, nausea and dizziness.
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A study of affected diplomats found they had experienced some form of brain injury, but could not determine the cause. There is no expert consensus on the syndrome's cause, but a co-author of the study considered microwave weapons "a main suspect". Others believe it may be the result of sonic boom attacks.
American officials have reported experiencing symptoms in Vietnam, Austria, Britain, Cuba and China, as well as the Washington DC area. At least 200 cases are under investigation, half of them involving spies, diplomats, soldiers and other US officials.
New guidelines are part of the federal government's plan to respond faster to potential cases, after it was discovered that a quicker response benefits the health of the victim and the investigation.
It is a shift in the US response after years of skepticism in some corners of government about whether the strange and unexplained health issues are real and connected.
"We need to believe our personnel who are coming forward," a senior administration official told US political website McClatchy DC.
"People are facing real symptoms," they said. "We are very conscious that people are experiencing something very real, and it is having a real negative effect on their health. And we're seeing better health outcomes the sooner we can respond to that."
The unnamed official firmly denied the idea that the syndrome is a "mass hysteria", saying "the physical effects we've seen in several cases are very, very real".