Rubio: Cuba accepts $100M aid but negotiated settlement unlikely

US Secretary of State Marco Rubio says Cuba has accepted an offer of $US100 million ($140 million) in aid but warned that a negotiated settlement between the countries is "not high".

Washington has stepped up its pressure campaign against the island's communist government in recent days, with US President Donald Trump pushing for "regime change" in Cuba.

Cuba has publicly only said that it was reviewing the aid offer made by Mr Rubio, and tensions have risen after the United States on Wednesday, local time, indicted the country's influential former president Raúl Castro on murder charges.

"They say they've accepted it," Mr Rubio told reporters in Miami about the aid offer.

But he added that the US would not allow the aid to flow into the country without controls.

"We're not going to do humanitarian aid that falls into the hands of their military company that they have. And then they take that stuff, and they sell it at the dollar stores and put the money in their pocket," Mr Rubio said.

Mr Rubio, a Cuban-American and vociferous critic of the government in Havana, said that the US was hoping to avoid the use of force.

The indictment in Miami of Mr Castro over a 1996 downing of two aircraft raised speculation that Mr Trump could use the charges as a pretext to attack the island and seize him.

"The president always has the option to do whatever it takes to support and protect the national interest and national security of the United States," Mr Rubio said.

"That said, our preference is always a diplomatic solution," he said.

"But if they have a change of heart, you know, we're here. And in the meantime, we'll keep doing what we need to do."

But Mr Rubio also made clear he believed Cuba posed a threat.

"Cuba not only has weapons that they've acquired from Russia and China over the years, but they also host a Russian and Chinese intelligence presence in their country, not far from where we're standing right now," he said.

"So Cuba has always posed a national security threat to the United States."

Supreme Court makes ruling over US assets

Mr Rubio's comments came on the same day the US Supreme Court ruled businesses could sue for compensation in a case that stemmed from the Cuban government's decision to seize American assets in 1959.

The justices, in an 8-1 ruling, set aside a lower court's decision to throw out the judgments against Carnival, Norwegian Cruise Line Holdings, Royal Caribbean Cruises and MSC Cruises, which were sued by a US company that had built the port facilities before the Cuban revolution.

The company filed suit under the Helms-Burton Act. This 1996 law allows US nationals who owned property in Cuba to sue anyone who "traffics in property which the Cuban Government confiscated on or after January 1, 1959."

Havana Docks Corporation, which built docks at Havana's port in the early 20th century, sued under the Helms-Burton Act, seeking compensation from the cruise lines whose ships have used the terminal.

Shortly after coming to power, former leader Fidel Castro nationalised and expropriated property held by US companies, including Havana Docks, which had a 99-year concession for the construction and operation of piers at the port of Havana, granted in 1934 by Cuba's government.

Cuba has never paid any compensation to Havana Docks, but the Helms-Burton Act gave the company an opening to sue for damages in US court.

The Supreme Court ruling means those four cruise operators face $US440 million in combined judgments.