Sir Keir Starmer and Volodymyr Zelenskyy said this is a "significant moment" for European security amid the continued fallout between the Ukrainian leader and Donald Trump.In a phone call this morning, the prime minister reiterated the UK's "ironclad support" for Ukraine and said he would discuss safeguarding Kyiv's sovereignty with the US president.Sir Keir also spoke with the European Commission president Ursula von der Leyen this morning, in which they agreed that Europe must "step up" for the good of collective security on the continent.Sir Keir will meet Mr Trump in the coming days for what will be a critical moment in the fragile relations between America and Europe after the US leader launched an extraordinary verbal attack on Mr Zelenskyy.Mr Trump labelled the Ukrainian leader a "dictator" and said the prime minister and French President Emmanuel Macron, both of whom will visit the White House next week, "haven't done anything" to end the war.Since Russia's full-scale invasion of Ukraine in 2022, the UK, France and other allies have been providing Kyiv with weapons and aid.Mr Trump has also suggested that Ukraine "started" the war.In a Downing Street readout of Sir Keir's call with Mr Zelenskyy, a spokesperson said: "The prime minister began by reiterating the UK's ironclad support for Ukraine and commitment to securing a just and enduring peace to bring an end to Russia's illegal war."The leaders reflected on recent developments in the conflict, as the third anniversary of the full-scale invasion approaches, and agreed on the need to continue working together with allies to achieve peace through strength."They agreed that this was a significant moment for the future of Ukraine and European security at large."Read more:Envoy says US is 'with' Ukraine - in contrast to Trump's remarksSombre Starmer and Europe confront emerging new world orderThe spokesperson added: "The prime minister repeated that Ukraine must be at the heart of any negotiations to end the war and that safeguarding Ukraine's sovereignty was essential to deter future aggression from Russia."Meanwhile, a Downing Street spokesperson said of Sir Keir and Ms von der Leyen's call: "Ahead of the three-year anniversary of Russia's barbaric illegal war, they discussed the need to secure a just and enduring peace in Ukraine, and agreed that Europe must step up for the good of collective European security."On Saturday afternoon, Sir Keir also spoke with Finland's president Alexander Stubb and discussed "the need to secure enduring peace for Ukraine and bring an end to Russia's illegal war", a Downing Street readout said."The leaders agreed the need for Europe to step up support to achieve this, and the prime minister said the UK is ready to play a role in future security guarantees."The US president's recent statements, which echo Russian president Vladimir Putin's narrative, and plans to have direct negotiations with Moscow about the end of the war in Ukraine have left European allies and Ukrainian officials worried.Mr Trump told US governors on Friday he "had very good talks with Putin" and "not such good talks with Ukraine".The US president also said a deal on minerals in Ukraine in exchange for weapons was close, but a source with knowledge of the negotiations told Sky News the Ukrainian president is not ready to sign and sees a "number of problematic issues" with the draft agreement.