Sir Keir Starmer and Volodymyr Zelensky have agreed on "the importance of refining" the UK's training offer for Ukrainian armed forces to "further bolster Ukraine's ability on the battlefield", Number 10 has said.
The Prime Minister and Ukrainian president spoke on Monday morning, and also agreed that what happens in the conflict in the coming months "matters to Nato".
According to a readout of the call issued by Number 10 on Monday, "President Zelensky reflected on the situation on the frontline in Ukraine and the need to ensure Ukraine could degrade Russian forces for the long haul.
"What happens in Ukraine in the coming weeks and months matters to Europe and Nato, and it was vital President Putin's ambitions fail in Ukraine, the leaders agreed."
The readout added: "The leaders discussed Operation Interflex, the UK-led training mission for Ukrainian recruits, and agreed on the importance of refining its offer to further bolster Ukraine's ability on the battlefield."
It comes amid speculation that the UK could be considering sending troops to Ukraine to help train its armed forces.
Last week, Defence Secretary John Healey said it is a "critical period" for Ukraine and pledged that the UK would be "stepping up" help.
Speaking to LBC during a visit to the Ukrainian capital Kyiv, Mr Healey said: "This is undoubtedly a critical period for Ukraine.
"Russians are putting Ukraine under pressure on the front line but Putin himself is showing signs of weakness, calling in North Korean troops to reinforce his own army, walking out on Assad and failing to defend his own positions in Syria."
He added: "I'm here discussing with Ukrainians and Ukrainian defence minister our joint plan for 2025 and I'm here to say, 'look, the UK is stepping up further the military help we'll give, and also stepping up further the UK international leadership to try and co-ordinate allies so that we can stand with Ukraine throughout 2025 and beyond'."
The call comes after Sir Keir's trip to the Joint Expeditionary Force (JEF) meeting last week, a military coalition of several European countries, during which he said that "Ukraine needs all the capability that it can get".