Calls by the White House for the UK and other Nato allies to invest in defence is “not a hostile thing”, US secretary of state Marco Rubio has insisted.

America’s top diplomat said it was because of the importance of the military alliance as a deterrent that members needed “to be as strong as they can possibly be”.

Britain and other Nato countries committed last year to hike defence spending to 5% of economic output by 2035 in the face of demands by Donald Trump, who claimed the US was bankrolling the security of other countries, and the burgeoning threats from Russia and China.

US defence secretary Pete Hegseth has also taken a combative approach towards Nato amid tensions over the Iran war, accusing some members of “free-riding”, while ordering a review of American military forces in Europe.

Stressing the need for “strong partners”, Mr Rubio noted many countries had “stepped up”, including Germany, while some, he did not name, “are dragging”.

The UK Government is under pressure to set out how it will meet the Nato spending target, amid reports Prime Minister Andy Burnham is looking at delaying an announcement on a key funding milestone.

Speaking in Portugal, Mr Rubio said: “People sometimes view this call to invest more in defence as a hostile thing. It is not a hostile thing.

“On the contrary, what we are arguing is that Nato is important. For Nato to matter, Nato has to be strong.

“For Nato to be strong, Nato’s member states need to be strong, and for Nato’s member states to be strong, they have to have capabilities.

“Now, every country is of a different size and proportion. Some countries can contribute more than others, but every country in Nato needs to be as strong as they can possibly be.

“Because the stronger the members of Nato are, the stronger Nato is. The stronger the alliance is and the stronger the deterrence is.”

He added: “Nato exists more than anything else as a defensive alliance that acts as a deterrent against hostility, and it has served us well for decades.

“If we want it to continue to serve us, we need to make sure that it is strong.”

Earlier, in Greece, Mr Rubio said: “The stronger the individual members of Nato are – both societally, economically, and militarily – the stronger Nato is. That’s what we want, and that’s what we’ve been asking for.

“I think there’s good signs in a lot of different places where that’s happening. Obviously, there are some countries that are dragging, and I know they have a lot of domestic pressure, but I think this is good for all of us.”

During his trip to Athens, Mr Rubio gave a keynote foreign policy speech in front of the Acropolis, in which he said moves to increase defence spending was a sign “Europe can awaken from its long slumber”.

Mr Rubio also said that while “passions” had strained ties between the US and Europe, they “must not be allowed to break them” as he highlighted the importance of a strong alliance in “confronting the threats of our age”.

The former Republican senator has been tipped as a frontrunner for his party’s nomination in the 2028 race for the White House.