A US union says some staff based at the US embassy in Cuba suffered permanent hearing loss as a result of mysterious 'sonic incidents' in Havana.

Both the Cuban and US authorities last month announced that investigations were underway after reports emerged that a number of US diplomats suffered serious hearing loss while stationed in Havana.

Reports have suggested a sonic device is likely responsible.

The American Foreign Service Association (AFSA) union now says staff had suffered "mild traumatic brain injury", permanent hearing loss, brain swelling, severe headaches, concussion, loss of balance and "cognitive disruption" as a result of what they described as "sonic harassment attacks".

It's the most detailed statement on the nature of the incidents, with most details having previously emerged through media reports.

In a statement, the union said: "AFSA strongly encourages the Department of State and the US Government to do everything possible to provide appropriate care for those affected, and to work to ensure that these incidents cease and are not repeated."

The US state department had previously said at least 16 US embassy employees were affected - but that has now increased to at least 19 Americans.

Spokeswoman Heather Nauert said authorities were continually revising assessments of the scope of the attacks, stressing that the investigation had not been completed.

She said: "We can confirm another incident which occurred last month and is now part of the investigation.

"We can't rule out new cases as medical professionals continue to evaluate members of the embassy community."

US officials have not publicly assigned any blame for what they have called 'incidents'.

At least one Canadian diplomat is also said to have been affected.

The US expelled two Cuban diplomats in response to the 'incidents' in Cuba, but officials have divulged few details of what exactly took place.

The reports emerged only two years after the US embassy in Havana re-opened, as part of improved diplomatic relations between the two states under Barack Obama.

Cuban authorities have previously denied any responsibility, insisting they would never allow "the Cuban territory be used for any action against accredited diplomatic agents or their families".

Additional reporting by IRN