The United States said Friday it holds Cuba responsible for investigating an apparent sonic attack that left several of its diplomats in need of medical treatment.
Secretary of State Rex Tillerson said the US mission in Havana had not been able to determine who was behind what he called "health attacks" on its staff.
But he warned: "We hold the Cubans responsible just as every host country has a responsibility for safety and security of diplomats in their country.
"We hold the Cuban authorities responsible for finding out who is carrying out these health attacks not just on our diplomats -- as you've seen there are cases with other diplomats as well."
On Thursday, Canada revealed that one of its diplomats had also fallen victim to the mysterious attack, which officials said seem to have been carried out with some kind of sonic device.
US personnel began experiencing ailments in late 2016, but that it was not immediately recognized that it could be anything other than an ordinary health issue.
US media have said the diplomats suffered hearing loss.
Cuba's foreign ministry said US officials had alerted it to the "alleged incidents" on February 17.
The State Department has not said how many of its diplomats were hurt, but in May it ordered two Cubans to leave the Cuban embassy in Washington.
Relations between the United States and Cuba were restored by then US president Barack Obama and his Cuban counterpart Raul Castro in 2015 after a half-century break.
But tensions mounted again after the detente was partly rolled back by Obama's successor Donald Trump.
In June, Trump tightened rules for Americans traveling to Cuba, banned ties with a military-run tourism firm and reaffirmed the existing US trade embargo.
The US embassy in Havana was closed in 1961 when diplomatic relations broke down between Washington and Fidel Castro's young revolutionary regime.