Rishi Sunak has urged European allies to maintain or boost military support for Ukraine over the coming year.
The PM kicked off a trip to the Baltic region by announcing the UK would send hundreds of thousands of rounds of artillery ammunition to Ukraine.
He also said there could not be peace talks until Russia withdrew its troops.
Last week, the BBC reported that the prime minister had asked for an assessment of the progress of the war in Ukraine.
A Whitehall source told BBC’s Newsnight programme: “Wars aren’t won [by dashboards]. Wars are won on instinct. At the start of this it was Boris (Johnson) sitting down and saying: ‘Let’s just go for this.’ So Rishi needs to channel his inner Boris on foreign policy though not of course on anything else.”
Downing Street has insisted Mr Sunak is strongly supportive of Ukraine.
During his trip to eastern Europe, the prime minister also met British troops serving in the Nato military alliance.
He served mince pieces to soldiers in Estonia and praised their “selfless dedication and bravery”.
Over the past year, the UK has doubled its presence in the country in response to the build up of Russian troops in Ukraine.
Earlier in the day, Mr Sunak had attended a meeting of the leaders of the Joint Expeditionary Forces (JEF) – a coalition of northern European countries, including Denmark, Estonia, Finland and Latvia.
Addressing the gathering in the Latvian capital Riga, Mr Sunak said it was an “incredibly important” time for them to reaffirm their commitment to Ukraine.
“We must be clear that any unilateral call for a ceasefire by Russia is completely meaningless in the current context.
“I think it would be a false call, it would be used by Russia to regroup, to reinforce their troops and until they have withdrawn from conquered territory, there can and should be no real negotiation.
“But what we can do is think about that time now, think about what we will do with regard to security assurances.”
He added that the group should concentrate on “degrading Russia’s capability to regroup and resupply”.
Ukraine President Volodymyr Zelensky also addressed the meeting, albeit over video link rather than in person.
He said providing Ukraine with a “100% air shield” would be “one of the most successful steps” countries could take against Russia.
He added that Russian missiles had left millions in his country without heat or power.
Following the summit, Mr Sunak met his Latvian counterpart Krisjanis Karins for talks, during which the pair agreed to work to accelerate trade between their countries.
Mr Karins tweeted: “As close and likeminded allies our countries have a shared commitment to regional security and a strong stance in support of Ukraine. Glad we also got to discuss strengthening of economic & high-tech links.”
Mr Sunak then headed to Estonia where he signed a technology partnership with the country’s Prime Minister, Kaja Kallas, aimed at improving cooperation on cyber security, connectivity and data.