The Cuban government says its forces have shot and killed four people on a US-registered speedboat that entered its waters and opened fire on a patrol boat.
It added that six others on the speedboat were injured.
The Cuban commander of the border patrol boat was also wounded, according to Cuba's interior ministry.
It said the injured were evacuated and are receiving medical attention.
The ministry said the incident is under investigation to establish exactly what happened.
US secretary of state Marco Rubio said no US government personnel were involved.
He added he was awaiting confirmation about whether those in the incident were US citizens, saying: "We're going to verify information independently and reach our own conclusions."
The Florida-registered speedboat came within one nautical mile of a channel on Falcones Cay, on Cuba's north coast, when it was approached by five members of a Cuban patrol unit, the interior ministry also said.
The crew of the speedboat then opened fire, wounding the commander of the Cuban vessel, the statement added.
Florida's attorney general James Uthmeier said he was ordering prosecutors to open a separate investigation with other state and federal law enforcement partners.
"The Cuban government cannot be trusted, and we will do everything in our power to hold these communists accountable," he wrote on X.
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Florida congressman Carlos Gimenez, a Cuban-American former mayor of Miami, demanded an "urgent" investigation into what he called a "massacre", adding US authorities "must determine whether any of the victims were US citizens or legal residents".
"This regime must be relegated to the dustbin of history!" he wrote on X.
None of the dead or wounded on the speedboat were identified, but Cuba said it was registered in Florida and provided the boat's registration number.
"Faced with the current challenges, Cuba reaffirms its commitment to protecting its territorial waters, based on the principle that national defence is a fundamental pillar for the Cuban state in safeguarding its sovereignty and stability in the region," the interior ministry statement said.
It comes amid a time of heightened tensions between the US and Cuba following increased pressure from Donald Trump's administration.
The two countries previously collaborated on combating drug smuggling and other crimes, but have ceased to do so.
The US has also blocked virtually all oil shipments to the island, piling pressure on its Communist-run government.
(c) Sky News 2026: Cuba says its forces have killed four on US-registered speedboat
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