Ukraine’s interior ministry announced Thursday that Russian troops had pushed from Belarus, north of Ukraine, into the highly radioactive Chernobyl exclusion zone, touching off a battle that risked damaging the cement-encased nuclear reactor that melted down in 1986.

“National Guard troops responsible for protecting the storage unit for dangerous radioactive waste are putting up fierce resistance,” said Anton Herashchenko, an adviser to the interior minister. Should an artillery shell hit the storage unit, Mr. Herashchenko said, “radioactive dust could cover the territory of Ukraine, Belarus and the countries of the European Union.”

Ukraine’s president, Volodymyr Zelensky, said on Twitter that Russian forces were trying to seize Chernobyl, the site of the worst nuclear accident in history. The site is now inside a protective zone covering about 1,000 square miles, including part of the shortest direct route from Belarus to Ukraine’s capital, Kyiv. “This is a declaration of war against the whole of Europe,” Mr. Zelensky said.