The incident comes amid increased tensions between the US and Cuba, which is facing a deepening fuel crisis that has been worsened by the US blocking oil shipments from Venezuela.

Cuban authorities said the speedboat had entered its territorial waters.

The island nation's interior ministry identified the six surviving passengers, who have since been detained, and one passenger who was killed.

They added that most of them "have prior records involving criminal and violent activity".

Handguns, assault rifles and improvised explosive devices were recovered on the speedboat, according to the statement, along with other tactical gear.

The interior ministry also identified an eleventh person they said had been arrested and had confessed to being part of the alleged plot.

In an earlier statement posted on X, the ministry said the Florida-registered vessel - with the registration number FL7726SH - was detected near Cayo Falcones, in the country's central Villa Clara province on Wednesday morning.

When a Cuban boat carrying five members of the ministry's border guard approached the vessel for identification, "the crew of the violating speedboat opened fire" and wounded the Cuban commander, the statement said.

"As a consequence of the confrontation, as of the time of this report, four aggressors on the foreign vessel were killed and six injured."

Those injured were evacuated and given medical assistance, the statement added.

Before the Cuban government released some of the passengers' identities, Rubio confirmed the boat was not carrying US government personnel and that an investigation was ongoing to "clarify" the event and what the passengers were doing in the area.

Rubio spoke from Saint Kitts and Nevis, where he had travelled to meet Caribbean leaders amid the Trump administration's push to ramp up pressure on Cuba's government, as well as other regional issues.

"We're going to find out exactly what happened, who was involved, and we'll make a determination on the basis of what we find out," he told reporters.

He vowed that US investigators would move "quickly" to gather the key facts, and that the US Coast Guard had travelled to the "vicinity" of the attack.

But he added that the US would not rely on information provided by the Cuban government, and that Washington would independently verify the facts of the case.

"It is highly unusual to see shootouts on the open sea like that. It's not something that happens every day," Rubio said.

The US-Cuba relationship has grown increasingly tense following the seizure by American forces of Venezuela's leader Nicolás Maduro on 3 January.

Venezuela has been a long-standing ally of Cuba, but in recent weeks the US has effectively stopped it from shipping oil to the Caribbean nation, and threatened tariffs on other nations if they do so.

Trump has urged Cuba to "make a deal" or face unspecified consequences.