Trump has threatened to pull US troops from Italy and Spain due to their opposition to the Iran war, with Madrid notably refusing to allow the use of its bases.

During a visit to Rome, Rubio was asked about that threat and the possibility of the US withdrawing from the NATO military alliance.

"If one of the main reasons why the US is in NATO is the ability to have forces deployed in Europe that we could project to other contingencies, and now that's no longer the case, at least when it comes to some NATO members, that's a problem, and it has to be examined," he told reporters.

He added that "ultimately that's a decision for the president to make", adding: "He hasn't made those decisions yet."

He said the announcement to withdraw 5,000 troops from Germany was "already pre-programmed" and signalled a return to 2022 levels.

"We have global obligations all over the world. We have troop deployments all over the world. And those are constantly shifting and changing," he added.

He said: "While we may be the most powerful country in the world, and we may have the most resources, our resources are not unlimited.

"We have to always assign resources around the world, including military resources, on the basis of what serves the national interest.

"That's a decision the president has to make, especially within the context of recent developments by some countries within NATO and the stance they've taken towards the United States.

"But no decision has been made on that yet."

Italy also recently refused authorisation for some US aircraft headed to the Middle East on a combat mission to land at its Sigonella base, a defence ministry source and Italian media said Tuesday.

Under the terms of agreements with the United States, some Italian bases can be used by US military aircraft, but only for logistical purposes.

Trump has threatened to slash US troop numbers in Germany and other European allies during both his White House terms, saying he wants Europe to take on greater responsibility for its defence rather than depending on Washington.

He now appears determined to punish allies who have failed to back the US in the Middle East war or contribute to a peacekeeping force in the crucial Strait of Hormuz waterway, which Tehran's forces have effectively closed.