US President Donald Trump has reiterated that Cuba is "next" after the military operation against Iran and said the Caribbean nation will fail soon.

"Cuba is going to be next," Trump told reporters aboard Air Force One en route to Washington, DC on Sunday.

He described Cuba as "a mess."

"It's a failing country, and they're going to be next. Within a short period of time, it's going to fail, and we will be there to help it out. We'll be there to help our great Cuban Americans out," he added.

'No problem' with Russian oil

Asked about a New York Times report that a Russian oil tanker would be allowed to reach Cuba, Trump said he has "no problem" with it.

"We don't mind having somebody get a boatload, because they have to survive.

"I told them, if a country wants to send some oil into Cuba right now, I have no problem, whether it's Russia or not," he said.

Asked if he is concerned that this would help Russian President Vladimir Putin, Trump said it would not.

"He loses one boatload of oil. That's all it is. It's fine. If he wants to do that and if other countries want to do it, it doesn't bother me much. It's not going to have an impact, Cuba's finished."

"They have a bad regime. They have a very bad and corrupt leadership, and whether or not they get a boat of oil, it's not going to matter. I'd prefer letting it in, whether it's Russia or anybody else, because the people need heat and cooling and all of the other things that you need," he added.