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.