Cuba poses a "national security threat" to the US, and the likelihood of a peaceful agreement is "not high", US Secretary of State Marco Rubio has said.

His comments come just a day after the US charged Cuba's former president Raúl Castro with murder over the 1996 downing of two planes, resulting in the killing of US nationals.

Rubio said Washington's preference was "a diplomatic solution" but warned that President Donald Trump had the right and obligation to protect his country against any threat.

Cuba's Foreign Minister Bruno Rodríguez accused Rubio of "lies" and said the island had never posed a threat to the US.

Speaking to reporters on Thursday, Rubio said diplomacy "remains our preference with Cuba", but added: "I'm just being honest with you, you know, the likelihood of that happening, given who we're dealing with right now, is not high."

He also accused Cuba of being "one of the leading sponsors of terrorism in the entire region", which Rodríguez vehemently denied in a post on X.

The Cuban foreign minister criticised Rubio for trying to "instigate a military aggression" and accused the US government of "ruthlessly and systematically" attacking his country.

Cuba is suffering from a fuel crisis exacerbated by an effective US oil blockade, while under pressure from the Trump administration to make a deal.

Its citizens have experienced extended blackouts and food shortages over the last few months.

Rubio said the country had accepted a US offer of $100m (£74.4m) in humanitarian aid.

Trump has repeatedly sought to exert pressure on Cuba and has openly discussed toppling its communist regime.

Wednesday's indictment of the former Cuban president is seen by some as reminiscent of Trump's seizure of Venezuelan President Nicolás Maduro in January.

Asked by reporters whether - and how - his government would get Castro to the US to face the charges, Rubio replied: "I'm not going to talk about how we're going to get him here, if we were trying to get him here, why would I say to the media what our plans are about that?"

Acting Attorney General Todd Blanche, who announced the charges in Miami on Wednesday, said the US "expect he will show up here, by his own will or another way".

On Thursday, Rubio also announced on X that the US had arrested Adys Lastres Morera, the sister of one of the top officials of a Cuban military-run conglomerate that controls most of the lucrative parts of the country's economy.

Morera was living in Florida "while also aiding Havana's communist regime", Rubio alleged. She was arrested by immigration enforcement and will remain in custody pending deportation proceedings.

Speaking to reporters in the Oval Office, Trump said Cuba was a "failed country" and that his administration was trying to help them "on a humanitarian basis".

He said Cuban-Americans "want to go back to their country" and help Cuba succeed.

"Other presidents have looked at this for 50, 60 years doing something and it looks like I'll be the one that does it, so I would be happy to do it," Trump said.