Keir Starmer had a room in stitches as he cracked a cheeky joke at the new US Ambassador's expence in Washington.

The PM, who arrived in the US for crunch talks with US President Donald Trump, enjoyed a playful dig at Peter Mandelson. The so-called Prince of Darkness has just taken up the huge job of being the UK's Trump whisperer in the US.

In a brief speech after landing, Mr Starmer told consular staff and dignitaries: "I've only just arrived, but already I can feel there's a real buzz around Washington right now.

"You can sense that there's a new leader in town that is a true one-off, a pioneer in business and in politics. Many people love him. Others love to hate him. But to us, he's just... Peter."

After the laughter died down, the PM went on: "Peter is the right person to help us work with President Trump, and to take the special relationship from strength to strength."

Mr Starmer is set to hold crunch talks in the Oval Office, with Ukraine top of the agenda. He will warn the President that unless the US provides a security guarantee for any peace deal, tyrant Vladimir Putin will strike again.

Ahead of the talks, Mr Starmer insisted Mr Trump was on side against Putin - despite his attacks on Ukrainian President Volodymr Zelensky, who he has accused of starting the war and branded a "dictator". In a make-or-break week, President Zelensky is expected to visit the White House on Friday to discuss a deal to give the US access to Ukraine's valuable minerals.

Mr Trump wants to claw back cash after complaining the US spent too much supporting Ukraine. But no security guarantees have been agreed - something Kyiv regards as critical. European leaders are pushing for a US "backstop" to any Ukraine peace agreement to stop Russia from attacking again.

But the US President poured cold water on these hopes, telling his Cabinet: "I'm not going to make security guarantees beyond very much. "We're going to have Europe do that, because we're talking about Europe as the next door neighbour."

Speaking to reporters on the way to Washington, Mr Starmer said: "The backstop is so important, that the security guarantee has to be sufficient to deter [Vladimir] Putin from coming again. Because my concern is if there is a ceasefire without a backstop, it will simply give him the opportunity to wait and to come again because his ambition in relation to Ukraine is pretty obvious for all to see."

Science Secretary Peter Kyle said the PM would be pushing for a US guarantee that it will commit to a peacekeeping mission. It comes after Mr Starmer said he was prepared to commit British troops if a deal is secured - so long as the United States does the same.

The Cabinet member declined to say what will happen if Trump refuses to give such an assurance. The US President said on Wednesday that he was not going to make security guarantees, sparking fears that tyrant Vladimir Putin could be appeased.

Mr Kyle told Sky News: "Ukraine is the number one item. There's a lot of this going to be on the agenda, of course. When do you come to these sorts of meetings both sides will have different priorities.

"The idea is for the first meeting is to build a relationship, is to establish the special relationship for the era that we are living in. But Ukraine is of particular pointed importance at this moment in time, because we're in the middle of this process where it looks like we will be negotiating a way forward. Hopefully that leads to peace."