The Prime Minister told fellow leaders there can be no peace talks with Russia until it has withdrawn from conquered regions of Ukraine.
Rishi Sunak urged them to ramp up their supply of air defence systems to the war-torn nation.
It comes as the UK announced a fresh supply of hundreds of thousands of rounds of artillery ammunition as part of its package of defensive aid for 2023, under a £250m contract.
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The Prime Minister said it was an "incredibly important" time for the Joint Expeditionary Force (JEF) to reaffirm its continued support for Kyiv, as Russian President Vladimir Putin escalates the conflict with "appalling" consequences for Ukraine.
Leaders of the UK-led JEF nations met in Riga on Monday to discuss Ukraine's defence needs while seeking to accelerate their own cooperation.
After the stop in the Latvian capital, Mr Sunak's whistlestop trip to the Baltic region continued with a visit to Estonia later in the day, where he met British troops and signed a new technology partnership.
Ukrainian President Volodymyr Zelensky addressed the gathering of the defence alliance via video link, appealing for a wide range of weapons systems including those to defend against aerial threats as Moscow continues to attack his country with Iranian drones.
"I call upon you to do everything to accelerate the defeat of the occupiers," he said through an interpreter.
"100% air shield for Ukraine, that would be one of the most successful steps against Russian aggression and this step is required right now."
He said millions were left without heat and power after Mr Putin's forces hit critical infrastructure in and around the country's capital with kamikaze drones overnight.
Mr Sunak urged the other JEF nations to sustain or boost their current offers, and to provide Kyiv with more air defence systems, artillery and armoured vehicles.
He continued: "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"I think it would be a false call, it would be used by Russia to regroup, to reinforce their troops and until they have withdrawn from conquered territory, there can and should be no real negotiation.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."
The UK is already Europe's leading provider of defensive aid to Ukraine, including sending multiple launch rocket systems and recently, 125 anti-aircraft guns.
More than 100,000 rounds of ammunition have also been provided by the UK since February, with the deliveries reportedly directly linked to successful operations to retake territory in Ukraine.
The Prime Minister also said the JEF alliance must focus on "degrading Russia's capability to regroup and resupply", including being "very strong about calling out" Iran for sending drones to Moscow.
At the summit's close, Mr Sunak met Latvian Prime Minister Krisjanis Karins for one-on-one talks, during which he praised the group's resolve in countering Russian aggression as "heartening".
Mr Karins later tweeted: "As close and like-minded 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."
The Prime Minister then travelled to Tallinn, using the short flight to hold a bilateral meeting with his Estonian counterpart Kaja Kallas who joined him on his plane.
Ms Kallas tweeted: "Thank you for your commitment to our defence.
"Today we also agreed to take our close digital partnership to a new level."
The agreement will look to strengthen the countries' ties across health, education, cyber security, data and connectivity.
In Estonia, which shares a 182-mile border with Russia, Mr Sunak met UK and Nato troops serving on the alliance's eastern flank.
The JEF comprises Denmark, Estonia, Finland, Iceland, Latvia, Lithuania, the Netherlands, Norway and Sweden.
After the summit, Mr Sunak said: "We talked specifically about the support that we're all collectively and individually providing to Ukraine.
"In the United Kingdom, we have committed to maintaining or increasing our military support to Ukraine next year. We need to make sure that we get that right."
He said the JEF was an example of a "constructive partnership" remaining after Brexit.