Switzerland on Thursday sent out official invitations to more than 160 delegations ahead of June's Ukraine peace conference - though Moscow was not among the invitees.
The peace talks are scheduled to take place on June 15-16 at the Bürgenstock resort on Lake Lucerne. Global leaders are expected to be in attendance.
"Russia has not been invited at this stage," a Swiss statement said.
Moscow has said several times in public that Russia would not participate in the event, the Swiss Foreign Ministry said.
"Switzerland is convinced that Russia must be involved in the course of this process. A peace process without Russia is unthinkable," the statement said.
The conference is not aimed at involving Russia, but seeks to bring together countries that have a shared understanding of the steps towards a solution, Ukrainian Foreign Minister Dmytro Kuleba told Foreign Policy, a US magazine.
That would be followed by communication with Russia, and Moscow could be involved in negotiations, as the war cannot be ended without the participation of both sides, said Kuleba.
"We don't understand what kind of milestone this peace conference is supposed to be," Kremlin spokesman Dmitry Peskov said in Moscow.
"How can we talk about a serious conference with serious results without Russia's participation?"
Russia's stance carries weight because China is calling for a peace conference that is acceptable to both sides, Russia and Ukraine. It is unclear whether China will join the Swiss event.
Switzerland's aim, however, is precisely to generate understanding for Ukraine's position among states that have so far backed Russia or taken a neutral stance such as China, India, Brazil and South Africa.
Switzerland is organizing the meeting at Kiev's request, with the aim of developing a pathway to a lasting peace more than two years after the full-scale Russian invasion of Ukraine.
Discussions are to be held on the basis of the peace plan Ukrainian President Volodymyr Zelensky presented to the United Nations in late 2022.
Kiev has demanded that the perpetrators of alleged war crimes in Ukraine be brought to justice. It is also seeking security and territorial guarantees.
In the autumn of 2022, Zelensky drew up 10 points that included the withdrawal of Russian troops, reconstruction and reparations, and the prosecution of war criminals.
Moscow, however, has asserted Russia's claimed right to rule over its neighbour, arguing that at most the future of western Ukraine is unclear.
Ukraine has been resisting the ongoing Russian invasion since February 2022.
Moscow's forces annexed the Crimea peninsula illegally in 2014, and Russia has claimed the Ukrainian regions of Donetsk, Luhansk, Zaporizhzhya and Kherson as its territory since the latest invasion.
Zelensky has also been pleading for more support and ammunition from Western partners as Russian forces push hard at the front lines and target infrastructure, particularly power plants.