Prime Minister Keir Starmer of Britain met with President Volodymyr Zelensky of Ukraine in Kyiv on Thursday, a show of solidarity four days before the inauguration of Donald J. Trump.
The trip to Ukraine, Mr. Starmer’s first since he became prime minister over the summer, comes amid a flurry of diplomacy between Kyiv and its European allies, who are watching to see whether Mr. Trump maintains American support for Ukraine in the war against Russia.
Britain is the third-largest provider of military aid to Ukraine, after the United States and Germany, and Downing Street said that Mr. Starmer and Mr. Zelensky planned to sign a “100-year partnership” between their two countries.
The agreement will include a particular focus on maritime security and promises to strengthen collaboration on technology, health care and education. It follows a narrower security agreement signed a year ago in response to the invasion that President Vladimir V. Putin of Russia ordered in 2022.
“Putin’s ambition to wrench Ukraine away from its closest partners has been a monumental strategic failure,” Mr. Starmer said in a statement. “Instead, we are closer than ever.”
Mr. Trump has said that he plans to quickly bring an end to the war, but he has not said how he will do so. Marco Rubio, his pick for secretary of state, said this week that both Russia and Ukraine would have to make concessions in any negotiations.
Kyiv’s official position is that it will not negotiate directly with Mr. Putin, a stance that Ukraine’s foreign minister, Andriy Sybiha, confirmed on Wednesday. But Mr. Zelensky has said that he hopes to meet with Mr. Trump at the earliest opportunity.
On Thursday, Valery Zaluzhny, Ukraine’s ambassador to Britain and former commander in chief, accompanied Mr. Starmer during the visit. He said the partnership would deepen security ties and bolster collaboration between the two countries.
Mr. Zelensky said at an allies meeting in Germany this month that he planned to discuss with Mr. Starmer “in the coming weeks” the possibility of Britain’s sending peacekeeping forces to Ukraine. The Ukrainian leader has also presented the idea to other European leaders, but they have met it with skepticism.
Ukraine has been receiving support in other ways from other European allies.
Germany’s defense minister, Boris Pistorius, said on a visit to Kyiv on Tuesday that he was optimistic that Berlin would find a way to send an additional 3 billion euros (about $3.1 billion) in military aid. But Germany is facing its own political upheaval after the government collapsed a month ago.
Mr. Zelensky discussed continued French military support for Ukraine with President Emmanuel Macron in a phone call this week, according to Mr. Zelensky’s office, and a Latvian delegation visited Ukraine on Tuesday to discuss military and humanitarian support.
And on Wednesday, Mr. Zelensky was in Warsaw, where he met with Poland’s prime minister, Donald Tusk, and its president, Andrzej Duda. Both expressed support for Ukraine’s bid to join NATO, a prospect that the military alliance has been reluctant to fully endorse, despite offering significant support to Kyiv.
Ukraine has increased its efforts on the diplomatic front as Russia’s offensive continues in the Donetsk region of eastern Ukraine.
The last of the modest gains Ukraine made during its summer 2023 counteroffensive have been wiped out, according to an assessment on Wednesday from DeepState, an analytical group with ties to the Ukrainian military, after Russian forces captured the village of Neskuchne.
Moscow’s forces are also making progress in their attempts to surround the key city of Pokrovsk, also in eastern Ukraine, from the south.
As Mr. Starmer was visiting Kyiv, the Ukrainian capital was placed under air-raid alert. Shortly afterward, a loud boom was heard in the skies above the city. Vitali Klitschko, the mayor, said that air defenses were operating and warned residents to stay in shelters.
Naralia Novosolova and Cassandra Vinograd contributed reporting.