Secretary of State Rex Tillerson has ordered a new inquiry into health problems suffered by diplomats serving at the US embassy in the Cuban capital Havana.
President Donald Trump has previously blamed the Cuban government for the mystery illnesses.
Reports suggest sonic attacks were to blame but nothing has been proven.
Cuba says there have been no sonic attacks against US embassy staff in Havana.
It describes the allegations as a “political manipulation” aimed at damaging bilateral relations.
Over a period of 18 months, 24 Americans including diplomats and family members suffered a range of symptoms including hearing loss, dizziness, fatigue, cognitive issues and sleep loss.
The problems were so prevalent the US decided to pull out most of its staff in the autumn, leaving just a skeleton operation.
America expelled 15 Cuban diplomats, saying that Havana had failed to protect its employees, but Cuba said the move was “unjustified”.
The US government also suspended visa processing in Cuba indefinitely.
So far, investigators have been unable to pinpoint the cause of the illnesses.
BBC Washington correspondent Gary O’Donoghue says this fresh inquiry – on top of investigations being conducted by agencies such as the FBI – is likely to strain relations even more following President Trump’s decision to back away from the policy of rapprochement begun by his predecessor.
Last year, Cuban Foreign Minister Bruno Rodriguez dismissed allegations of any kind of attack on US embassy staff as “totally false”.
At least one Canadian has experienced similar symptoms.
Mystery in Havana
Late 2016: US embassy staff and at least one Canadian begin to notice symptoms
May 2017: US expels two Cuban diplomats for Cuba failing to protect its diplomats
August: US says 16 employees have been treated but attacks seem to have stopped. A Canadian diplomat in Havana is treated for hearing loss
Early September: US says attacks are continuing and 19 staff members have now been hurt
29 September: Washington pulls out diplomatic staff, warns US citizens not to visit and says 21 embassy employees now injured
3 October: US expels Cuban diplomats from Washington