World leaders gather in Switzerland on Saturday for a summit aimed at pressuring Russia to end its war in Ukraine, but the absence of powerful allies of Moscow such as China will blunt its potential impact.

Dozens of allies of Ukraine will take part in the summit, but China is staying away after Russia was frozen out of proceedings on the grounds it had dismissed the event as a waste of time and had no interest in attending.

Without China, hopes of isolating Moscow have faded, while recent military reverses have put Kyiv on the back foot. The war in Gaza between Israel and Hamas has also diverted attention from Ukraine.

The talks are expected to focus on broader concerns triggered by the war, such as food and nuclear security and freedom of navigation, and a draft of the final declaration identifies Russia as the aggressor in the conflict, sources said.

"The summit risks showing the limits of Ukrainian diplomacy," said Richard Gowan, UN Director at the International Crisis Group. "Nonetheless, it is also a chance for Ukraine to remind the world that it is defending the principles of the UN Charter."

Russian President Vladimir Putin said on Friday Russia would end the war in Ukraine only if Kyiv agreed to drop its NATO ambitions and hand over the entirety of four provinces claimed by Moscow - demands Kyiv swiftly rejected as tantamount to surrender.

Putin's conditions apparently reflected Moscow's growing confidence that its forces have the upper hand in the war.

Moscow casts what it calls its special military operation in Ukraine as part of a broader struggle with the West, which it says wants to bring Russia to its knees. Kyiv and the West reject this and accuse Russia of waging an illegal war of conquest.

Ukraine's President Volodymyr Zelensky (right) steps toward a limousine after his landing in Obbuergen. AP

Switzerland, which took on the summit at the behest of Ukrainian President Volodymyr Zelensky, wants to pave the way for a future peace process that includes Russia.

But geopolitical splits over the deadliest European conflict since World War Two have dogged the event, and Zelenskiy has even accused Beijing of helping Moscow undermine the gathering, an accusation China's foreign ministry denied.

China had said it would consider taking part, but ultimately declined because Russia would not be there.

"It's clear that at the moment, in geopolitical terms, for China the special relationship with Russia takes precedence over any other consideration," said Bernardino Regazzoni, a former Swiss ambassador to China.

Around 90 countries and organizations have committed to the two-day gathering due to take place at the Buergenstock, a mountaintop resort in central Switzerland. The summit has also had to contend with an alternative plan floated by China.

US Vice President Kamala Harris and the leaders of France, Germany, Italy, Britain, Canada and Japan are among those due to attend. India, Turkey and Hungary, which maintain friendlier relations with Russia, are also expected to join.

Russia, which sent tens of thousands of troops into Ukraine in February 2022, has described the idea of a summit without it as "futile".

Supporters of Ukraine are marking the Swiss talks with a series of events in the nearby city of Lucerne to draw attention to the war's humanitarian costs, with a demonstration planned to call for the return of prisoners and children taken to Russia.

European officials privately concede that without support from Moscow's main allies, the summit's impact will be limited.

"What can (Zelenskiy) hope for out of it?" said Daniel Woker, a former Swiss ambassador. "Another small step forward in international solidarity with Ukraine as the victim of Russian aggression."

Reuters