British Prime Minister Keir Starmer told leaders gathered Sunday for a summit on the war in Ukraine that they need to step up and continue to support Kyiv and meet a “once in a generation moment” for the security of Europe.

“Even while Russia talks about peace, they are continuing their relentless aggression,” Starmer said at the opening of the meeting.

Starmer, flanked by Ukrainian President Volodymyr Zelenskyy and French President Emmanuel Macron, said the three of them have agreed to work on a plan to stop the fighting, and then take it to the United States, which has discussed brokering a peace deal.

“We need to agree what steps come out of this meeting to deliver peace through strength for the benefit of all,” he said. “So, let’s get started.”

The meeting, which has taken on greater importance in defending the war-torn ally, has been overshadowed by the extraordinary scolding of Zelenskyy by President Trump, who blasted him Friday at the White House as being ungrateful for U.S. support against the invasion by Russia.

Starmer told CBS News partner BBC that he does not trust Russian President Vladimir Putin but does trust Mr. Trump.

“Do I believe Donald Trump when he says he wants lasting peace? The answer to that is yes,” he said.

Starmer said there are “intense discussions” to get a security guarantee from the U.S.

“If there is to be a deal, if there is to be a stopping of the fighting, then that agreement has to be defended, because the worst of all outcomes is that there is a temporary pause and then Putin comes again,” Starmer said. “That has happened in the past, I think it is a real risk, and that is why we must ensure that if there’s a deal, it is a lasting deal, not a temporary pause.”

The three essentials Starmer listed for a successful peace deal were: arming the Ukrainians to put them in a position of strength; including a European element to guarantee security; and providing a “U.S. backstop,” to prevent Putin from breaking promises.

“That’s the package. All three parts need to be in place, and that’s what I’m working hard to bring together,” Starmer said.

Starmer hosted the meeting at Lancaster House, a 200-year-old mansion near Buckingham Palace, following a charm offensive last week to persuade Mr. Trump to put Ukraine at the center of negotiations and tilt his allegiances toward Europe.

Leaders from Germany, Denmark, Italy, Netherlands, Norway, Poland, Spain, Canada, Finland, Sweden, the Czech Republic and Romania were at the summit. The Turkish foreign minister, NATO secretary-general and the presidents of the European Commission and European Council were also in attendance.

Meetings in recent days had provided some hope — until Zelenskyy visited the White House.

Visits to the Oval Office by Macron, who had declared his visit a “turning point,” and Starmer were seen as steps in the right direction. The meetings were cordial and Mr. Trump even took a gentler tone toward Ukraine, though he would not commit to providing U.S. security guarantees and maintained Europe would need to provide peacekeeping troops.

European Union top diplomat Kaja Kallas told “Face the Nation with Margaret Brennan” in an interview taped Friday that the visits to the White House were coordinated with a message that the fight is “much broader” than just Ukraine’s sovereignty.

“It’s about freedom of the free world, really. It is about the world where international law applies and the world where might does not make right,” she said. “It is clear that Russia attacked Ukraine. There is one aggressor and one victim. And we need to really make sure that Russia doesn’t attack again. And for that, we need to concentrate our efforts. I mean putting politically and economically pressure on Russia to stop this war, but at the same time also help Ukraine to defend itself.”