MOSCOW (Reuters) - The Kremlin said on Saturday it wanted a definitive response from Ukraine to Russian President Vladimir Putin's offer of a three-day ceasefire next week, criticising the reaction so far as ambiguous and historically wrong.

Putin on Monday declared a three-day ceasefire to mark the 80th anniversary of the victory of the Soviet Union and its allies over Nazi Germany in World War Two.

The Kremlin said the 72-hour ceasefire would run on May 8, May 9 - when Putin will host international leaders on Moscow's Red Square, including Chinese President Xi Jinping - and May 10.

Ukrainian President Volodymyr Zelenskiy appeared to rule out such a brief ceasefire earlier on Saturday, saying he was only ready to sign up to a ceasefire that would last at least 30 days, an idea Putin has said needs a lot of work before it could become a reality.

Zelenskiy also said that Ukraine, given the continued war with Russia, could not guarantee the safety of any foreign dignitaries who came to Moscow for the May 9 parade.

Russia's Foreign Ministry said his comments amounted to a threat, while Dmitry Medvedev, deputy chairman of Russia's Security Council, said nobody could guarantee that the Ukrainian capital of Kyiv would survive to see May 10 if Ukraine attacked Moscow during the May 9 celebrations.

Kremlin spokesman Dmitry Peskov convened a special conference call after Zelenskiy's comments.

He told reporters that Putin's three-day offer had been a test to assess Kyiv's readiness to search for a peaceful settlement to end the war.

"The reaction of the Ukrainian authorities to Russia's initiative to introduce a ceasefire is a test of Ukraine's readiness for peace. And we will, of course, await not ambiguous but definitive statements and, most importantly, actions aimed at de-escalating the conflict over the public holidays," Peskov said.

He accused the Ukrainian authorities of espousing "neo-Nazism," an allegation Kyiv has repeatedly rejected as false, and of not considering the victory over Nazi Germany to be important enough to mark properly.

Peskov also commented on media reports that Ukrainian soldiers will take part in World War Two commemorations in Britain, calling the move "sacrilege".

(Reporting by Dmitry Antonov in Moscow; Editing by Andrew Osborn)