President Volodymyr Zelensky on Saturday said Ukraine will not adhere to a three-day cease-fire proposed by Russia, saying such a short truce would not help in negotiations for a lasting peace.

Ukraine in March accepted a proposal from the Trump administration for a 30-day cease-fire, to be followed by talks on matters including the location of an armistice line, steps to safeguard a nuclear power plant occupied by the Russian army and deployment of a possible European peacekeeping force.

On Saturday Mr. Zelensky told reporters in Kyiv that the Russian counteroffer of a three-day cease-fire covering the anniversary of the end of World War II in Europe, an important political holiday in Russia, was merely a “theatrical show.”

He said Russia’s offer was intended to “create a soft atmosphere of an exit from isolation” for President Vladimir V. Putin of Russia, and to assuage security worries of foreign leaders and dignitaries attending a Victory Day parade in Moscow on May 9. The Chinese president, Xi Jinping, has accepted an invitation to attend the parade.

“We are simply either at war, or Putin shows he is ready to stop shooting as a first step to ending the war,” Mr. Zelensky said. Ukraine’s offer of a 30-day truce remains on the table, he added, saying Ukraine would be willing to start that truce before the May 9 holiday if Russia accepted it.