The Havana Syndrome first arose in 2016 in some US embassy staff in Havana (Alex Sandoval)

The CIA has concluded that US diplomats that have been suffering mysterious headaches and nausea in a phenomenon that's been called "Havana Syndrome" were not targets of a global campaign by a foreign power.

A number of US news websites cited officials briefed in a CIA intelligence assessment on developments regarding the syndrome. The "Havana Syndrome" first surfaced among staff in the US embassy in Cuba in 2016, when US and Canadian diplomats reported they suffered from severe headaches, nausea, and possible brain damage after hearing high-pitched sounds.

Many different diplomats and intelligence officials reported similar experiences since then, such as in Australia, Austria, China, Colombia, Germany and Russia.

The CIA assessment says that although the agency does not rule out foreign involvement in about two dozen cases for which no answers have been found, it has found plausible explanations in hundreds of other cases.

Kremlin rebuffs allegations of Russia’s involvement in ‘Havana Syndrome’

In November last year, CIA Director William Burns warned Russia of consequences if it was linked to the experiences. Kremlin Spokesperson Dmitry Peskov rejected the spy chief's allegation.

"This topic was in no way raised by Burns in conversations with political interlocutors here in Moscow and it was not touched upon with the head of state [Vladimir Putin]. As for the content of Burns’s conversation with his colleagues in the special services, I have nothing to say here - this is not public information. The only thing is that here we can surely reject any hints, assumptions, or statements about the alleged involvement of the Russian side in these cases - we have nothing to do with this," Peskov assured reporters.