Europe mustered a spirited show of support for Ukraine on Friday, as Britain assembled the “coalition of the willing” to promote a boycott of Russian oil and gas, a possible loan to Ukraine using Russian sovereign assets and accelerated arms shipments.

Seeking to capitalize on yet another shift in President Trump’s view of the war, President Volodymyr Zelensky of Ukraine joined Prime Minister Keir Starmer of Britain and leaders from Denmark, the Netherlands and NATO in London. Twenty other leaders joined by videoconference, including Japan’s new prime minister, Sanae Takaichi — the first time a Japanese leader has taken part in these sessions.

For the moment, Europe has the wind at its back. Mr. Trump’s decision to impose sanctions on Russia’s two largest oil companies, Rosneft and Lukoil, was a striking shift in his position after a contentious meeting with Mr. Zelensky a week ago.

The European Union followed with its own latest round of sanctions, while also pledging to continue funding Ukraine’s financial and military needs. Britain, one of Ukraine’s most enthusiastic backers, announced that it would speed the delivery of an extra 140 air-defense missiles after production had ramped up faster than expected.

Mr. Starmer praised Mr. Trump for the sanctions, saying it was an important step in the campaign to pressure Russia. “We must now maintain this pressure,” he said at a news conference after the meeting.

President Vladimir V. Putin of Russia “continues to stall and play for time,” Mr. Starmer added. “We are determined to act now, to dial up the pressure on Putin, and finally bring him to the negotiating table in good faith.”

Mr. Zelensky said Russia was carrying out a “campaign of terror against our energy system,” tormenting the Ukrainian people as winter approaches. “Russia’s goal hasn’t changed: They want to break us,” he said, standing alongside Mr. Starmer and the other leaders.

Britain framed this meeting as an effort to fortify Ukraine for the winter fighting months. It and France jointly lead the so-called coalition of the willing, which Mr. Starmer and President Emmanuel Macron started in March. Its prospects have waxed and waned, in part because of Mr. Trump’s erratic views — he pivots between pressuring Mr. Zelensky and criticizing Mr. Putin — and in part because of Europe’s own fitful struggle for unity.

While the European Union followed the Trump administration in imposing energy sanctions on Russia, European leaders were unable to agree this week on a plan to use frozen Russian assets to finance a huge loan to Ukraine. Belgium, which holds the bulk of the assets, voiced concerns about legal liability.

Mr. Starmer pressed his fellow leaders to move ahead with the loan plan, though Britain, having left the European Union in 2020, no longer exerts much influence on negotiations in Brussels. Prime Minister Mette Frederiksen of Denmark said the countries still had to resolve technical questions about the assets, but she hoped there would be a decision on the matter by Christmas Eve.

Reflecting Europe’s sometimes ambivalent attitude toward Russia, Ms. Frederiksen acknowledged that Europe’s decision to boycott Russian oil companies might have come too late. But she said, “It doesn’t mean we haven’t done anything during this war when it comes to Russian oil.”

Before joining Mr. Starmer for a one-on-one meeting in Downing Street, Mr. Zelensky met with King Charles III at Windsor Castle, west of London. The king has met the president three times this year in what is viewed as a symbolic show of British solidarity.

Britain has frequently played the role of soothing ally as Mr. Zelensky navigates the crosscurrents of Mr. Trump. In February, after an explosive meeting with the American president in the Oval Office, Mr. Zelensky’s first stop was London, where he and Mr. Starmer began devising a plan to repair the breach with the White House.

Once again, Britain was a quick destination for Mr. Zelensky after his tense meeting with the president in Washington a week ago. Mr. Trump, fresh off what he described as a positive phone conversation with Mr. Putin, piled new pressure on Mr. Zelensky to hand over territory to Russia to end the hostilities.

The difference this time was that Mr. Trump quickly reversed himself, scrapping plans to meet Mr. Putin in person, though he did accede to the Russian leader’s request to not send Tomahawk long-range missiles to Ukraine. Then, on Wednesday, Mr. Trump announced the sanctions on the Rosneft and Lukoil.

Mr. Putin on Thursday called the move “an unfriendly act,” and said that while the sanctions would hurt Russia’s economy, Moscow would never make any concessions under pressure.

The other leaders attending Friday’s meeting joined Mr. Starmer in heaping praise on Mr. Trump for that decision. NATO’s secretary-general, Mark Rutte, who also met with the president recently in Washington, said, “It’s up to each ally to decide which weapons they want to deliver to Ukraine.” He noted that although Mr. Trump had ruled out Tomahawk missiles, the United States was still supplying a “wide range of weapons.”

While European leaders try to keep up the economic and diplomatic pressure on Russia, their military commanders are continuing to put together plans to secure Ukraine in the event that Russia agrees to a cease-fire and peace negotiation.

“We, as the military, have to plan and think about and develop something which can then be used in a reasonable manner by the political masters,” Lt. Gen. Alexander Sollfrank, the commander of the German army’s joint force command, said in an interview earlier this month.

A creative, well-designed military plan, General Sollfrank said, could function as a “kind of leverage” for diplomatic efforts to end the war.

Steven Erlanger contributed reporting from Berlin.