Keir Starmer has denied being snubbed by Donald Trump at the G7 in France after the two did not have a bilateral meeting at the summit.

The prime minister, who did meet the US president in a series of discussions involving other leaders, said he had a series of “very productive, very good conversations” with the US president. He added that Britain was ready to play its “full part” in opening the strait of Hormuz after a peace deal between Iran and the United States.

The prime minister said it was vital for British interests that the strait of Hormuz was opened “as quickly as possible” after the breakthrough. “The strait being closed in the way that it has had an impact on our economy, had an impact on every household across the country, so we’ll play our full part in relation to that,” he said, later adding that the UK could be involved in demining the vital shipping route.

Earlier on Tuesday, Starmer was caught on a hot mic asking: “Are they having a meeting?” while Trump, Emmanuel Macron and Volodymyr Zelenskyy were absent.

Starmer also appeared to indicate that there would be no new money for defence in the further delayed defence investment plan after the resignation of John Healey, who quit as defence secretary over the funding being made available last week. Starmer said Labour had overseen the biggest sustained increase in defence spending since the 1980s, increasing funding from 2.3% to 2.6% of GDP.

He said the new defence secretary, Dan Jarvis, was looking at where he wanted the money to be spent. “He will now want to look at his own priorities and capabilities, and he may want to take a slightly different approach as to what those capabilities and priorities are now he’s looking at the plan,” he said.

Starmer said he had spoken to Macron on Monday about next steps in deciding any role played by European allies in opening the strait, adding: “We do want to make sure that where we’ve got capability and demining is an obvious example where we’re coordinating that we agree a way forward with the United States and others to get the strait of Hormuz open as soon as possible.”

Trump said on Monday he did not believe the US would “need much help” in ensuring the free passage of shipping in the strait, but it was not “a bad idea to have a ship or two” from other countries based in the strategic waterway.

Speaking at the summit on the first day of full meetings at the Hôtel Royal in Évian, Starmer said his conversations with Trump had been focused on Iran and Ukraine.

He said the two men had not discussed the potentially problematic issues of defence spending and a ban on social media for the under-16s in the UK, but a “very honest and frank conversation” during dinner had focused on Iran. “So, contrary to what you may put to me, [it was] actually a very constructive, very good discussion,” he said.

Starmer, who announced new sanctions on Russia to target finance networks and expand the number of vessels targeted as part of Moscow’s shadow fleet carrying oil or liquified natural gas (LNG) to more than 600, said there was a “mood change” in the debate about Ukraine.

“[There is a] real unity in the G7 on Ukraine, a real sense that things are changing, that Ukraine is now taking territory, which it has done in the last few months, rather than defending territory, that the impact of sanctions is quite great on Russia,” he said.

“We’ve obviously put new sanctions in today, and there was a shared understanding around the table [ …] that the mood in Moscow is changing.”

Starmer, who may be attending his last international meeting before a potential leadership battle after the Makerfield byelection on Thursday, said he would not “walk away” and would contest any challenge.

“We got a landslide victory in 2024 for a five-year mandate for this Labour government,” he said. “They said it’s not possible to turn the Labour party around. It’s not possible to win an election. It’s not possible if you do win an election to invest in your public services and stabilise the economy – wrong every time, and that’s why I intend not to walk away from this, but to carry on with what I was elected to do, which is to serve this country, bring back the change that people desperately need in their lives.”