Six Cuban exiles were being held under guard in hospital on Thursday after what Havana described as a heavily-armed attempt 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 US-Cuban tensions; Donald Trump tightened economic sanctions on Cuba since capturing Venezuelan president Nicolas Maduro, a key Cuban ally, on 3 January.
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.
Florida-based politicians 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, about 150 miles east of Havana and the capital of the province where the incident occurred.
The hospital was under heavy guard. Security personnel who stopped Reuters journalists near the entrance confirmed the suspects were held there but provided no further details.
Cuban government and US embassy officials in Havana did not immediately respond to requests for comment.
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 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 inside 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. Government opponents may have been emboldened by the overthrow of Maduro and a tightening US oil blockade that has exacerbated severe energy shortages.
