Sir Keir Starmer has said he wants to use the Nato summit to “reset” relations with world leaders and send a message to Russian President Vladimir Putin.

The Prime Minister held talks with Ukraine’s Volodymyr Zelensky before arriving at the summit and will later meet US President Joe Biden at the White House.

Sir Keir said he used his talks with Mr Zelensky to stress that the change of government in the UK “makes no difference to the support that we will provide”.

The summit was also a chance for the new Labour Government “to reaffirm our commitment to Nato” on its 75th anniversary, he said.

It was an opportunity “to discuss with President Zelensky what further support he needs and to use our opportunity here with our allies to make sure that that support is agreed”, Sir Keir said.

“And to reinforce, in a sense as a message to Putin, the resolve of Nato, bigger now than it’s ever been, more united than it’s ever been and absolutely clear-eyed about the threat of Russian aggression,” the Prime Minister added.

The summit marks Sir Keir’s debut on the world stage, less than a week after the Labour landslide which installed him in No 10.

He told reporters in Washington DC his talks with world leaders were “an opportunity to make sure that those relationships are reset, for me to be able to say that our position on the world stage – leading on issues like defence and security, on climate change, and on energy – are so important”.

The summit is expected to confirm what US and European officials have described as Ukraine’s “irreversible” path to Nato membership.

But the alliance’s mutual defence pact means Ukraine will not join while it is at open war with Russia, as that could result in all-out conflict between the bloc and Mr Putin’s forces.

The summit comes as the alliance debates how to “Trump proof” its future and its support for Ukraine in case former US president Donald Trump is returned to the White House in November’s election.

He has been a vocal critic of European nations not doing enough to fund their own defence and has threatened to withdraw support for Ukraine.

In the UK, the Government will launch a sweeping defence review next week which will set out a “road map” to spending 2.5% of gross domestic product on defence.

But Sir Keir refused to guarantee that he will meet his flagship commitment on defence spending within his first term in office, despite the “cast iron” promise to get there.

Speaking to reporters, the Prime Minister was repeatedly pressed on whether the goal of spending 2.5% of GDP would be reached within his first term.

He said: “We are committed to the 2.5%, as I have said before the election and I say again after the election.

“That is obviously subject to our fiscal rules, but the commitment is there.

“The strategic review will take place, that will happen next week, and we will set out the details of that.

“The manifesto commitment was that it would take place within a year, I would like it to be quicker than that if I’m honest and we’ll set out the details about how we are going to do it.”

Officials have described 2.5% as a “cast iron commitment” but the announcement that a strategic defence review will be launched next week did not include any timetable for ramping up spending.

Before his election defeat, Rishi Sunak had committed to reach 2.5% by 2030 at a total cost of £75 billion over six years.

Nato members have an official goal of spending 2% of GDP on defence and 23 members are now thought to have reached that level.

But in the context of this year’s US election, Sir Keir acknowledged more needed to be done, particularly on locking in support for Ukraine.

The Prime Minister confirmed that decisions on the use of UK-supplied long-range Storm Shadow missiles were for the Ukrainian armed forces to make, indicating they would be permitted to strike against targets within Russia.

The UK military aid was “for defensive purposes but it is for Ukraine to decide how to deploy it for those defensive purposes”, Sir Keir said.

The Russian strike on a children’s hospital in Kyiv on Monday was a “tragic backdrop to this summit” and “strengthens the resolve” of Nato against Mr Putin, he said.

President Zelensky wrote on X, formerly Twitter, that he was “grateful to the UK for its unwavering support for Ukraine and our people”.

He added: “I thanked Prime Minister Starmer for all the UK’s military and financial assistance provided to our country.

“This morning, I learned about the permission to use Storm Shadow missiles against military targets in Russian territory.

“Today, we had the opportunity to discuss the practical implementation of this decision.”

The summit is also an introduction for the Prime Minister’s wife Victoria to international diplomacy.

She will take part in a series of engagements with other leaders’ wives and husbands, led by US First Lady Jill Biden.