The US state department has expelled two diplomats from the Cuban embassy in Washington following a series of unexplained incidents in Cuba that left US officials there with physical symptoms that one official said includes potentially permanent hearing loss.

Spokeswoman Heather Nauert said that the Cubans were asked to leave the US on 23 May after Americans in Cuba “reported incidents which have caused a variety of physical symptoms”, causing them to leave the island.

Nauert said the first of the incidents was reported in late 2016 and that they had continued. She would not say what the symptoms were except that they were not life-threatening. Nauert also declined to provide details about the incidents. The FBI and Diplomatic Security Service are investigating.

However, other US officials said that the symptoms included hearing loss.

One person familiar with the US investigation said investigators were looking into whether elements of the Cuban government placed sonic devices that produce non-audible sound inside or outside the residences of roughly five US embassy staffers with the intent of deafening them. That individual and the US officials weren’t authorized to discuss the investigation publicly and spoke on condition of anonymity.

Cuba employs a massive state security apparatus that keeps hundreds and possibly thousands of people under constant surveillance. US diplomats are among the most closely monitored people on the island. It’s virtually impossible for anyone to take action against an American diplomat without an element of the Cuban state being aware.

However, the person familiar with the US investigation said investigators were looking into whether the individuals were harmed outside the regular chain of command of the Cuban government.

The officials said the staffers all arrived in Havana in the summer of 2016. Like all foreign diplomats in Cuba, they lived in housing owned and maintained by the Cuban government.

In the autumn, officials said the affected diplomats and their spouses began to experience symptoms of hearing loss so severe and puzzling that an investigation was launched, and it was determined that they were at risk. They were allowed to leave Cuba, the officials said.

No children were affected, but at least some of the adults who were are believed to have suffered permanent hearing loss, according to the officials. They said the Cuban government had denied any involvement.

Although she would not provide details, Nauert said that investigators did not yet have a definitive explanation for the incidents but stressed they take them “very seriously” and are working to determine their “cause and impact”. She said the department had reminded Cuba of its international obligation to protect foreign diplomats.

The spokeswoman declined to comment on whether other countries have suffered the same problem in Cuba and are joining in the investigation. “We’ve been clear in our response and our concern ... We remain in regular contact with the Cuban government. They are providing some guidance, some assistance with the investigation.”

Harassment of US diplomats in Cuba is not uncommon and dates to the restoration of limited ties with the communist government in the 1970s. But the use of sonic devices to intentionally harm diplomats would mark a new phase in harassment.

The Associated Press contributed to this report