He will reaffirm Britain's enduring support for the Western military alliance and Ukraine's fight against Russian aggression, he told reporters on the tarmac at London Stansted Airport.

"But today and tomorrow and the next day is all about standing together with our allies, discussing practically how we provide further support for Ukraine, and send a very, very clear message to Putin that we will stand against Russian aggression wherever it is in the world," he said.

Starmer, 61, told a meeting of his top team that the summit represents an opportunity to "reset relationships, reinvigorate our unshakeable commitment to the alliance and demonstrate the strength of Britain on the world stage".

The visit kicks off a whirlwind of international diplomacy in Starmer's first two weeks in power, with the UK also hosting a European leaders' conference next week.

"It will be an opportunity for him to learn and get to know other leaders as much as to communicate any particular messages," foreign policy expert James Strong told AFP.

Britain's previous Conservative government was one of Ukraine's staunchest allies, providing money, weapons and troop training to help it repel Russia's invasion.

Starmer has pledged continued support for Kyiv under Labour, and is expected to reaffirm that message in person to Ukrainian President Volodymyr Zelensky at the NATO meeting.

Starmer's Defence Secretary John Healey has already visited Ukraine since last Thursday's election, and Foreign Secretary David Lammy has been visiting European NATO members.

Labour is committed to the alliance and wants to match the Conservatives' promise to raise defence spending to 2.5 percent of GDP, above the NATO target of two percent.

"We can expect lots of talk about 'business as usual'," added Strong, a lecturer at Queen Mary University of London.

While Starmer will stress continuity on the main foreign policy issues, he will also be keen to signal a reset in relations with allies that were soured by Brexit.

Labour has pledged closer cooperation with European neighbours, including on bilateral deals with France and Germany but also on agreements with the EU bloc as a whole.

- 'Special relationship' -

"We can expect to hear a lot of talk about improving relations, about being a more reliable partner, and above all about being more stable and predictable," said Strong.

The trip also presents an early opportunity for Starmer to build a rapport with US President Joe Biden and cement the so-called UK-US special relationship.

Starmer's centre-left Labour party is more closely aligned with Biden's Democratic Party than the Conservatives, which could help, but the trip comes at a sensitive time for the US president.

Following a poor debate performance, Biden, 81, is under pressure to make way and allow a younger Democrat to take on Republican rival Donald Trump in November's presidential election.

Starmer will be mindful that he may have to deal with the isolationist Trump from January next year.

"(He) will want to demonstrate his rock-solid commitment to the UK's alliance with America, and to send a strong signal about the resilience of the 'special relationship', whatever lies ahead," Sophia Gaston, head of foreign policy at the right-wing Policy Exchange think-tank, told AFP.

A point of contention between Starmer and Biden could be the Israel-Hamas war in Gaza, with Labour viewed as more pro-Palestinian than Washington.

The leaders are also likely to discuss policy towards China, including on trade, commerce and technology.

Starmer will then host the European Political Community summit at Blenheim Palace, near Oxford, in central England, on July 18, with France's president Emmanuel Macron and Germany's leader Olaf Scholz expected to attend.

Labour wants "an ambitious" security pact with the European Union.

Olivia O'Sullivan, director of the UK in the world programme at the Chatham House international affairs think-tank, told AFP the two summits "provide an opportunity to put more meat on the bones of this proposal".

Starmer may also want to flesh out the shape of a post-Brexit trading deal, after his introduction to life on the global stage in Washington.