Six Cuban exiles were being held under guard in hospital on Thursday after what Cuba has described as an attempt by heavily armed men to infiltrate the country by speedboat.
Four others aboard were killed when Cuban forces returned fire on the Florida-registered vessel.
The incident comes amid heightened tensions between the US and Cuba – Donald Trump has tightened economic sanctions on Cuba since capturing the Venezuelan president Nicolas Maduro, a key Cuban ally, on 3 January.
The US secretary of state Marco Rubio said his government would independently investigate the incident, and that the US embassy in Havana was seeking access to the survivors to determine if any were US citizens or permanent residents. He said it was not a US operation and that no US government personnel were involved.
Politicians in Florida have pledged to conduct their own probes, saying they distrust the Cuban account.
Cuba said the six survivors were receiving medical attention. At least some appeared to be in custody at the Arnaldo Milian Castro Provincial Clinical Surgical Hospital in Santa Clara, the capital of the province where the incident occurred and about 150 miles east of Havana.
The hospital was under heavy guard. Security personnel who stopped Reuters journalists near the entrance confirmed the suspects were being held there but provided no further details.
Cuban government and US embassy officials in Havana did not immediately respond to requests for comment.
The Cuban president Miguel Diaz-Canel vowed to defend the country against aggression.
“Cuba will defend itself with determination and firmness against any terrorist and mercenary aggression that seeks to affect its sovereignty and national stability,” he said.
“Cuba does not attack, nor threaten,” he added, repeating a common Cuban response to decades of US economic sanctions and the virtual oil blockade imposed by Mr Trump in January.
Cuba said the suspects came from the US dressed in camouflage and armed with assault rifles, handguns, homemade explosives, ballistic vests and telescopic sights.
An additional Cuban suspect was detained on Cuban territory in connection with the plot, and the boat commander was also wounded, government officials said.
Many Cuban exiles, largely concentrated in Miami, have long dreamed of overthrowing Cuba’s Communist government or seeing it fall.
Exile paramilitaries have attempted or carried out acts of sabotage in the past. Opponents of the government may have been emboldened by the overthrow of Mr Maduro and a tightening US oil blockade that has exacerbated severe energy shortages.
