Washington has expelled two Cuban diplomats after US embassy staff suffered severe hearing loss, attributed to a covert sonic device.

In a story straight from the pages of a Cold War spy novel, a bizarre series of events in the autumn of last year saw US diplomats stationed at a newly reopened embassy in Havana complain of feeling unwell.

Their mysterious symptoms were said to be like those of a concussion, which can include headaches, nausea, vomiting, dizziness, confusion, memory loss and ringing in the ears.

Some of the diplomats fell so ill they were forced to cancel their tours early and return to the United States.

"When [staff] started reporting symptoms it took time to figure out what it was," said Heather Nauert, a spokeswoman for the US State Department. "We take this extremely seriously."

Suspecting foul play, US investigators from agencies including the FBI began work trying to find the source.

After several months they concluded that diplomats had been exposed to an advanced device that emitted sound beyond the acoustic range audible to humans.

Investigators believe it was deployed either inside or outside their residences.

It was not immediately clear if the device was a weapon used in a deliberate espionage attack on diplomats or had some other purpose.

US officials weren't authorised to discuss the investigation publicly and spoke on condition of anonymity to Associated Press.

They said about five diplomats, several with spouses, had been affected and that no children had been involved. Two of the group who returned to the US for treatment are said to now rely on hearing aids due to partial deafness.

While the US did not directly accused Cuba of involvement, Nauert said the US retaliated by expelling two Cuban diplomats from their embassy in Washington on 23 May.

It's a move that suggests the US believes Cuba has at best been negligent in its duty to protect foreign diplomats on its soil.

"We requested their departure as a reciprocal measure since some US personnel's assignments in Havana had to be curtailed due to these incidents," she said. "Under the Vienna Convention, Cuba has an obligation to take measures to protect diplomats."

Investigators are probing whether a third country may have been involved.

The Cuban government said in a lengthy statement late Wednesday (9 August) that "Cuba has never permitted, nor will permit, that Cuban territory be used for any action against accredited diplomatic officials or their families, with no exception."

The statement from the Cuban Foreign Ministry said it had been informed of the incidents on 17 February and had launched an "exhaustive, high-priority, urgent investigation at the behest of the highest level of the Cuban government."

It said the decision to expel two Cuban diplomats was "unjustified and baseless".

The incident comes after a thaw in US-Cuba relations under President Barack Obama saw official diplomatic channels resume in 2015.

It followed decades of tensions between the two countries, with the CIA involved in numerous outlandish assassination plots against former Communist leader Fidel Castro.

This included plots to kill Castro with poisoned cigars, exploding seashells and a James Bond-style poisoned pen.

The use of acoustic devices against diplomats is thought to be unprecedented, however.

Even so, weaponised sound waves go beyond science fiction.

US military and private companies have researched and developed the use of sonic weapons to incapacitate, injure or even kill targets.

In 2009, police in Pittsburgh used a Long Range Acoustic Device (LRAD) to emit high-pitched, pain-inducing sounds at crowds protesting during the G20 Summit. It led to the city paying out more than $200,000 in settlements, with one bystander suffering permanent hearing loss.

In a weapon that mimics the sound gun used by Tom Cruise in Hollywood blockbuster Minority Report, a US company says it has also developed "sonic bullets" that can stop people in their tracks with an intense beam up to 145 decibels – 50 times the human threshold of pain.

"[For] most people even if they plug their ears, it will produce the equivalent of an instant migraine. Some people, it will knock them on their knees," LRAD founder Woody Norris told ABC News while explaining the device.