It's Zakir Starmer's first international trip as prime minister, and much of the focus of the talks will be on providing practical support for Ukraine in its battle against Russia.

But behind the scenes, pressure continues to mount on President Biden to step back from seeking a second term in office after last month's poor performance in a TV debate with Donald Trump.

In the last few minutes, a seventh senior Democrat says he should pull out of the race.

Well, let's join Sarah Smith live on Capitol Hill in Washington for us now.

Sarah.

President Biden hoped that he had stopped the rebellion within his party when he sent a very firmly worded letter to representatives here on Capitol Hill saying that he is the candidate for the Democratic Party and he wasn't pulling out of the race.

Well, it hasn't worked.

And as you say, we have yet one more Democrat, Mickey Sherrill of New Jersey, saying she doesn't think that he is the person best placed to beat Donald Trump in November and that that's what really matters.

Because after all, Joe Biden is the one who has said that this election is more important than any other, that democracy in America is at stake.

Well, now his candidacy is at stake at the same time, of course, as he's preparing to host that important NATO summit here in Washington.

Joe Biden has made it clear he is not going anywhere, refusing to even consider stepping aside as a Democratic candidate for president.

Yet on Capitol Hill, Democrats were huddled behind closed doors discussing his future.

No comment.

I'm going to give you my first no comment, my current no comment.

I don't have anything to add beyond what I said on summit.

You convince President Biden can win this election?

Worst jobs in 100 years.

You remember the long lines to food banks?

A handful of Democrats have publicly called for Biden to step aside.

Others have expressed their support.

There is obviously still deep disagreement within the party over whether he should be their candidate.

I'm proud to be a member of the Democratic Party because it's a big tent party.

We listen to each other.

We respect each other.

And then we make decisions and we move forward.

You're always going to have people that have differing views because that's the American way.

That's the Democratic Party.

That's OK.

We just have to find a way to move together forward.

And at some point they have to shut up?

No, I wouldn't say that.

When members speak out and say they want President Biden to step aside as a candidate, is that hurting the party's re-election chances?

It could.

It could.

The Democratic Party leadership are trying to put an end to all this talk of changing their presidential candidate.

Are you confident that President Biden has what it takes to win in November and serve the next four years?

As I've said before, I'm with Joe.

Inside the White House, there's frustration that this speculation over the president's future continues.

We do want to turn the page.

You heard me say this last week.

We want to get to the other side of this.

We want to continue doing the work and that's what the president is going to do.

World leaders from over 30 countries are arriving in Washington for an important NATO summit.

One that will now be carefully watched as a major test of Joe Biden's fitness for office.

Sarah Smith, BBC News, Washington.