Keir Starmer has said it is in the best interests of the US to stay in Nato and that Europe must do more to support the alliance in light of the war in Iran.

The British prime minister, speaking at the end of a multi-stop trip around the Gulf to discuss the tentative ceasefire and options to fully reopen the commercially vital strait of Hormuz, pushed back against Donald Trump’s threats to leave the defence alliance.

The US president has repeatedly criticised European members of Nato, which largely disagreed with the attacks on Iran, for not participating more fully in the strikes. He included a threat to pull Washington out of the alliance altogether.

Starmer, speaking in Qatar at the end of a trip during which he also met leaders in Saudi Arabia, the United Arab Emirates and Bahrain, agreed with Trump’s calls for European members of Nato to spend more on defence, while insisting it was in the interests of the US to stay in the alliance.

“We’re very strong supporters of Nato and I’ve been making the argument for some considerable time that we need to do more,” he said. “It’s the single most effective military alliance the world has ever known.

“Do we Europeans need to do more? Yes. I’ve been making that argument for the best part of two years, to our European partners as much as anybody else. We continue to make that case and we will make that case.”

He added: “It is in America’s interests; it’s in European interests. Nato is a defensive alliance, which for decades has kept us much safer than we would otherwise have been.

“Do I think this will be a stronger European element to Nato? Yes, and I think we should step into that space. We’re already doing it, which is why we’re coordinating strategically with our partners in Nato.”

Starmer and Trump spoke on Thursday night. Talking to broadcasters in Doha, Starmer said much of the call was spent discussing how to ensure ships could safely pass through the strait of Hormuz, a key shipping route for oil and gas as well as other goods, which Iran in effect blocked after attacks by the US and Israel.

As well as ensuring the sea route is safe, Starmer has stressed the need to avoid the potential scenario of Iran charging levies on ships passing through. Earlier this week Trump mooted the idea of a “joint venture” between the US and Iran to set tolls.

Starmer, setting out his talks with the various Gulf leaders, said they shared this view and were concerned about the ceasefire, which is under threat from continued Israeli attacks on Lebanon and Iran warning it could retaliate.

“Obviously, the discussion moved very quickly to the ceasefire, a sense that it’s fragile, that more work is needed, that the strait of Hormuz has to be part of the solution, a very strong sense that there can’t be tolling or restrictions on that navigation,” he said.

Trump has appeared volatile and irascible as the Iran war has failed to produce regime change in Tehran or achieve any other tangible US goals. He has expressed this in insults to other world leaders, including Starmer, and threats to Iran, including threatening before the ceasefire that the country’s “whole civilisation [would] die” if it did not meet US demands.

John Healey, the UK defence secretary, said on Friday that rather than poring over Trump’s social media posts, those in the US should instead examine the UK’s actions in the Gulf, saying these “spoke for themselves”.

While the UK refused to participate in attacks on Iran, it contributed significantly to efforts to defend Gulf states from Iranian attacks, with UK planes and ground gunners shooting down a number of Iranian drones and missiles. UK bases were also available to US planes, if only for missions seen as defensive.

“Even in this current conflict, the basing permissions that we in the UK have agreed with the US have been invaluable to their military operations,” Healey said, adding: “If we focus on our actions rather than just simply the exchange of words and social media posts, then the fundamentals for me remain.”