US officials are still baffled after the country's diplomats in Cuba fell seriously ill due to what was first believed to be a covert sonic device.

Fresh details emerged this week indicating at least some of the incidents were confined to certain rooms, or even parts of rooms, with laser-like specificity.

Investigators say the facts and the physics don't add up.

One US diplomat recalled how he was awoken in his Havana hotel by a blaring, grinding noise. There was then silence when he moved just a few feet.

When he climbed back into bed, the agonising sound hit him again.

In several other episodes recounted by US officials, victims knew it was happening in real time, and there were strong indications of a sonic attack.

Some felt vibrations, and heard sounds – loud ringing or a high-pitch chirping similar to crickets or cicadas. Others heard the grinding noise. Some victims awoke with ringing in their ears and fumbled for their alarm clocks, only to discover the ringing stopped when they moved away from their beds.

The attacks seemed to come at night; several victims reported they came in minute-long bursts.

Yet others heard nothing, felt nothing, and were only aware they'd been affected when their symptoms arrived later.

"None of this has a reasonable explanation," Fulton Armstrong, a former CIA official who served in Havana long before America re-opened an embassy there, told Associated Press.

"It's just mystery after mystery after mystery."

Reports first emerged in the autumn of last year, and at least 21 US victims – suffering a range of symptoms, including hearing loss and speech problems – are said to have been affected.

The top US diplomat has called them "health attacks", with some embassy staff affected so badly they had to return back the their home country.

Some have even since been diagnosed with mild traumatic brain injury, damage to their central nervous system and permanent hearing loss.

It's a story that could come straight from the pages of a Cold War spy novel, and suspicion was first placed on a sonic weapon, and on Cuba.

It even prompted the US to expel Cuban diplomats from Washington last month, suggesting the US believed that at the very least the country had failed to fulfil its obligations under the Vienna Convention to protect foreign diplomats on its soil.

But the US has stopped short of directly accusing Havana, suggesting they believe those at the top of the Cuban government may know nothing of the suspected attacks.

Mark Feierstein, who oversaw the 2015 Cuba detente on President Barack Obama's National Security Council, also noted that Cuban authorities have been uncharacteristically cooperative with the investigation.

"Had they thought the Cuban government was deliberately attacking American diplomats, that would have had a much more negative effect," Feierstein said. "We haven't seen that yet."

Furthermore, the diagnosis of mild brain injury – considered unlikely to result from sound – has confounded the FBI, the State Department and US intelligence agencies involved in the investigation.

Some victims now have problems concentrating or recalling specific words, several officials said, the latest signs of more serious damage than the US government initially thought.

It has stumped FBI and US officials, who are now looking at whether a different device altogether may be to blame, perhaps planted by Cuba, a rogue faction of its security forces, or by a third country – like Russia.

They've left open the possibility an advanced espionage operation went horribly awry, or that some other, less nefarious explanation is to blame.

"The investigation into all of this is still under way. It is an aggressive investigation," State Department spokeswoman Heather Nauert said Thursday. "We will continue doing this until we find out who or what is responsible for this."

Cuba has denied any involvement.

"Cuba has never, nor would it ever, allow that the Cuban territory be used for any action against accredited diplomatic agents or their families, without exception," the Cuban statement said.