Sir Keir Starmer said he wants to send a “very clear” message to Vladimir Putin that the Nato summit is an opportunity for allies to stand together against Russian aggression.
He flew to Washington on Tuesday evening with his wife Victoria, after addressing a new cohort of 643 MPs in the House of Commons for the first time as Prime Minister.
The summit will be the Prime Minister’s debut on the world stage and provide an early opportunity to meet Joe Biden, who is hosting the gathering.
Support for Ukraine is set to be top of the agenda for the Washington summit.
Before departing at Stansted Airport, Sir Keir said: “My message is very, very clear.
“This Nato summit is an opportunity for allies to stand together, to strengthen their resolve, particularly in light of that appalling attack, against Russian aggression,” he said, referring to Russian missile attacks on Monday that damaged the country’s largest children’s hospital.
“I’m pleased to have the early opportunity to confirm and reaffirm Labour’s strong support, unshakeable support for Nato.
“We are a founding member, it’s now the 75th anniversary but this is an opportunity to send that message in relation to Russian aggression wherever.
“But that attack … was appalling so that message is even more important than it was before.”
Nato leaders began gathering on Tuesday as the alliance marks its 75th anniversary, with US President Joe Biden hosting Mr Starmer at the White House.
But with Mr Biden’s position under threat, both from Republican rival Donald Trump and Democrats concerned about the 81-year-old seeking a second term, the US leader may be preoccupied with domestic concerns.
The Prime Minister’s schedule mean he isn’t joining the Nato summit until the main working sessions begin on Wednesday, after parliament reconvened on Tuesday following the election.
Sir Keir appeared in the Commons as Prime Minister for the first time as the formal process of re-electing Sir Lindsay Hoyle as Speaker took place.
He praised Britain’s “most diverse” parliament and paid tribute to the new Mother of the House, Diane Abbott, who was re-elected as Labour MP Hackney North and Stoke Newington, following a Labour row over her selection as the candidate.
He said she “has done so much in her career over so many years to fight for a parliament that truly represents modern Britain. We welcome her back to her place”.
Ms Abbott made her own tearful speech to parliament, where she warned the country was heading into “very tumultuous times”.
Ukraine will be the key talking point as Mr Biden hosts Sir Keir at the White House on Wednesday, with the president’s press secretary saying the former will “underscore the importance of continuing to strengthen the special relationship between the United States and the United Kingdom”.
The leaders will also have the opportunity to discuss UK-US cooperation on advancing a free and open Indo-Pacific, reaching a ceasefire and hostage release deal in Gaza, Iran and the threat of Houthi rebels to commercial shipping.
White House Press Secretary Karine Jean-Pierre said: “The leaders will also discuss furthering our bilateral cooperation in areas such as protecting advanced technologies and developing climate and clean energy solutions.”
Defence Secretary John Healey has already visited Ukraine to promise more artillery guns and shells along with nearly 100 Brimstone missiles.
With the prospect of a sceptical Mr Trump entering the White House after November’s election, there is also likely to be a focus on getting European Nato members to spend more on defence.
The Government has an ambition to increase spending to 2.5% of gross domestic product, a measure of the size of the economy, when the nation’s finances allow.
But during the election campaign Sir Keir was criticised by his Tory rival Rishi Sunak, who in Government had set out a plan to reach 2.5% by 2030.
Ahead of the Nato summit, Sir Keir said it would be “an opportunity for me to have further discussions with some of the leaders I’ve already spoken to and some that I’m due to speak to”.
At his first press conference as Prime Minister on Saturday he said it was an “important summit” for the transatlantic alliance and gave him the chance to “make clear our unshakeable support of Nato”.
