Sir Keir Starmer will have his first face-to-face meeting with US President Joe Biden in the White House later.
Speaking on the plane, Sir Keir told reporters he hopes the bilateral in Washington DC will be a chance to talk about Nato and the so-called UK-US special relationship.
“This is obviously a very special relationship we have between the UK and the US,” he said, adding there was “a special aspect when it comes to defence and security for obvious reasons, including our commitment to Nato”.
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He also told journalists that the UK had a “cast-iron commitment” to spend 2.5% of national income on defence but avoided putting a deadline on when that would be met.
A review of defence capabilities, to be ordered by the new government, will set out a roadmap towards reaching the target.
Defence Secretary John Healey said the review would help the government “get to grips with the threats we face, the capabilities we need for the future, the state of our armed forces”.
Pressed on when the target would be met, he said the government had to have “the chance to open the books and get to grips with the terrible state of public finances”.
He said the defence review could not be done in weeks but would be “done properly” and “done at pace”.
The Conservatives, who during the election pledged to meet 2.5% by 2030, have said failure to set out a timetable would “cast serious doubt” on the government’s commitment to security.
Alongside Mr Healey, the prime minister has been joined on his trip by Foreign Secretary David Lammy and minister for European relations Nick Thomas-Symonds.
After arriving in Washington, Sir Keir and Mr Thomas-Symonds met German Chancellor Olaf Scholz.
The prime minister also met Ukrainian president Volodymyr Zelensky.
Sir Keir reaffirmed his support for Ukraine, saying: “There’s a change of government but no change of approach.”
He told BBC Political Editor Chris Mason he discussed with President Zelensky “what further support he needs” and he said he would use the Nato summit “to make sure that support is agreed”.
The UK prime minister said he also wanted to send “a message to Putin” about the resolve of Nato, that it was “more united than it has ever been” and “absolutely clear-eyed about the threat of Russian aggression”.
Mr Zelensky thanked the UK for being “with us from the very beginning of the war”.
While the prime minister is in Washington, more than 300 new MPs continued to be sworn in at Westminster, ready for the state opening of parliament next week, when the government’s programme is laid out in the King’s Speech.
During his flight to the US, Sir Keir was asked about whether he believed age restrictions the government plans to introduce in the House of Lords, forcing peers to retire at the age of 80, should be applied across the board.
This would affect Labour peers such as figures like Labour’s Lord Alf Dubs, who came to the UK as a child fleeing the Nazis and has long campaigned on refugee rights, and is 91-years-old.
Sir Keir said: “We’ve got 800-plus members of the House of Lords – it’s simply too big.
“We need to reduce it, so it doesn’t reflect on how other elected representatives are chosen in other countries, it’s to do with the size of the House of Lords.”
