On May 16, 2025, British Prime Minister Keir Starmer called Russia's position presented at the talks with Ukraine in Istanbul "clearly unacceptable," accusing Moscow of seeking to prolong the conflict. This was reported by Sky News, quoting Starmer, who stressed that Russia "not for the first time" demonstrated an unwillingness to compromise. The statement came after Starmer's meeting with Ukrainian President Volodymyr Zelensky in Ankara and a telephone conversation with US President Donald Trump, during which the parties discussed coordinating further steps. According to the prime minister, the allies are "closely coordinating responses" to increase pressure on Russia and support Ukraine.

The Istanbul talks, the first direct contacts between Russia and Ukraine since March 2022, ended on May 16 with an agreement on a 1000-for-1000 prisoner swap, the largest of the conflict. However, as The Economist's Oliver Carroll reported, Russia demanded the withdrawal of Ukrainian troops from four regions and hinted at the possibility of occupying the Kharkiv and Sumy regions, which drew sharp criticism. The head of the Russian delegation, Vladimir Medinsky, according to Reuters, declared his readiness for a "long struggle", comparing the conflict to the 21-year Northern War with Sweden.

Starmer's stance reflects the line of Western allies. The leaders of Britain, France, Germany and Poland have previously rejected Russia's demand to stop arms sales to Ukraine in exchange for a ceasefire, calling it an "ultimatum," The Guardian reported. Trump, despite backing the talks, has threatened to abandon peace efforts if there is no progress, adding to the pressure on both sides.