Leaders of the so-called "coalition of the willing" are set to travel to Ukraine for further talks, Volodymyr Zelensky has said.
The Ukrainian president said in a video address to the Joint Expeditionary Force summit in Oslo, Norway, today that he would host leaders of the coalition tomorrow.
He said there was "serious work ahead", adding: "We need this coalition and we need it to be strong enough to guarantee security the way we all agree on."
Military officers from around 30 countries have been involved in drawing up plans for the coalition which would provide a peacekeeping force in the event of a ceasefire being agreed between Russia and Ukraine.
Meanwhile, Europe announced its support for a special tribunal to prosecute President Vladimir Putin and other senior Russian officials for the crime of aggression against Ukraine - a show of unity on a day when Moscow marks its 'victory day'.
At a meeting in Ukraine's western city of Lviv, ministers from almost 20 European nations gave their political sign-off to the tribunal, welcoming the completion of the technical work required to set it up.
The tribunal will be set up within the framework of the Council of Europe, the continent's leading human rights watchdog that was formed after World War II to uphold rights and the rule of law.
European nations have stepped up their efforts since US President Donald Trump authorised sanctions in February against the International Criminal Court, created to prosecute war crimes when member states are unwilling or unable to do so themselves.
Russia denies its troops have committed atrocities in Ukraine since its forces launched a full-scale invasion in February 2022. Ukraine says Russian troops have committed many thousands of war crimes.
Foreign ministers from the so-called 'core group' of at least 37 countries signed the "Lviv statement", a document marking the conclusion of work done to draft the necessary legal instruments for the tribunal. The tribunal could start operating next year.
"This tribunal will ensure that those most responsible for the aggression against Ukraine are held accountable," EU foreign policy chief Kaja Kallas told reporters.
Mr Zelensky said it was a moral duty for Europe to hold Russia accountable for the war.
"A strong tribunal for the crime of aggression can - and must - make any potential aggressor think twice," he said in a video address to the meeting.
'Good step'
European Commissioner for Justice and Democracy Michael McGrath attended the meeting in Lviv, along with Ms Kallas.
In a statement issued by the European Commission, Mr McGrath said that the Commission endorsed the special tribunal for crimes of aggression against Ukraine.
"We are leading the charge for justice, to bring perpetrators of this illegal war of aggression to account and to assert our unwavering commitment to let the rules-based order triumph over aggression, arbitrary rule, and impunity," said Mr McGrath.
Minister of State for International Development and Diaspora Neale Richmond represented Ireland at the meeting.
Mr Richmond said that by endorsing the creation of the special tribunal, the EU was "collectively defending a world where law, not military might, is the final arbiter of what is just and right".
Earlier, Ukrainian Prime Minister Denys Shmyhal led ministers to a military cemetery in Lviv to honour some of the tens of thousands of Ukrainians killed during more than three years of war against Russia.
The images were in stark contrast to the show of military might in Moscow, where Mr Putin and Chinese President Xi Jinping attended a parade to mark the 80th anniversary of the World War II defeat of Nazi Germany.
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Drawing a parallel with the allied victory in World War II, British foreign minister David Lammy said 80 years later "in Ukraine we are (again) on the frontlines of that fight for freedom ... sending a powerful message to Putin and his cronies and those that stand with him that freedom will prevail".
European nations are trying to ramp up the pressure on Mr Putin to accept an unconditional 30-day ceasefire, as proposed by the US. Separately, Britain has announced a new package of sanctions on Russia's so-called shadow fleet.
Ukraine has been pushing for the creation of a special tribunal since early in the conflict, accusing Russian troops of committing many thousands of war crimes, but is also intent on prosecuting Russians for orchestrating the 2022 invasion.
The ICC has issued an arrest warrant against Mr Putin over the deportation of Ukrainian children. Moscow says the warrant is meaningless, and "null and void".
But the ICC cannot prosecute Russian officials for the crime of aggression in Ukraine because Russia is not a member of the Rome Statute, which created the court, and Ukraine was not a full member at the time of the invasion.
For the ICC to prosecute the crime of aggression, members must also sign an additional amendment.
Russia hit villages 220 times during ceasefire - Ukraine
Russia hit eight Ukrainian settlements near a frontline 220 times during ceasefire, governor of the Zaporizhzhia region Ivan Fedorov said on the Telegram messaging platform.
The villages in Zaporizhzhia were hit by 150 drones and 70 artillery rounds in the past 24 hours, he said.
Ukraine's foreign minister said yesterday that Russia had repeatedly violated its own three-day ceasefire hours after it began and called the initiative a "farce", while Moscow said Kyiv had continued fighting.
There was, though, a drop-off in combat activity after the ceasefire announced by Mr Putin came into force in the early hours of yesterday, with a respite in the drone and missile attacks that had rattled Ukrainian cities earlier this week.
"Predictably, Putin's 'Parade ceasefire' proves to be a farce," Ukrainian Foreign Minister Andrii Sybiha said on X, referring to the truce which coincides with a 9 May parade on Moscow's Red Square to commemorate the end of World War II.
"Russian forces continue to attack across the entire frontline," Mr Sybiha wrote.
"From midnight to midday, Russia committed 734 ceasefire violations and 63 assault operations, 23 of which are still ongoing."
He said Ukraine was notifying the United States and European states about Russia's actions.
The Russian defence ministry, cited by Interfax news agency, said that Ukraine, in turn, had carried out 488 attacks on Russian targets and twice tried to break through the border in the Kursk region.
The two sides did not immediately comment on each other's battlefield reports, which Reuters could not independently confirm.
A late night report issued by the General Staff of Ukraine's military nearly 24 hours in the proclaimed ceasefire said 154 clashes had been recorded.
Russian forces, it said, had launched one missile attack and 15 air strikes.
The governor of southeastern Zaporizhzhia region said a Russian drone had struck a car in the south of the region, killing a passenger.
In central Poltava region, the head of the regional military administration said air defence units had downed a missile deployed by Russian forces.
The missile damaged private homes, but caused no casualties.
