OTTAWA, Canada (AFP) – A Canadian diplomat in Cuba has been treated for hearing loss, an official said Thursday, after revelations that a number of US embassy staff in Cuba suffering similar symptoms were forced to leave Havana.

US officials have told CNN that its employees may have been attacked with a sonic device, outside the range of audible sound, either inside or outside their residences in Havana.

It was not immediately clear if Cuba was to blame — or another nation, officials told the network. “We are aware of unusual symptoms affecting Canadian and US diplomatic personnel and their families in Havana,” Canadian foreign ministry spokeswoman Brianne Maxwell said.

“The government is actively working — including with US and Cuban authorities — to ascertain the cause.”

On Wednesday, the US State Department said the symptoms were first reported late last year.

“Some US government personnel who were working at our embassy in Havana, Cuba … reported some incidents which have caused a variety of physical symptoms,” State Department spokeswoman Heather Nauert said.

“We don’t have any definitive answers about the source or the cause,” she said.

Two Cuban diplomats were expelled from Washington in May over the incident, Nauert announced.

“We had to bring some Americans home or some Americans chose to come home as a result of that. And as a result of that, we’ve asked two Cubans to leave the United States and they have,” she told reporters.

Cuba said it had objected to the expulsion of its officials, while also urging the United States to work together to shed light on the incidents in Havana.

Relations between the United States and Cuba were restored in 2015 after a half-century break by then US president Barack Obama and his Cuban counterpart Raul Castro.

But the rapprochement was partially rolled back by Obama’s successor Donald Trump, who in June announced tightened rules for Americans travelling to Cuba, banned ties with a military-run tourism firm and reaffirmed the existing US trade embargo.