KHARKIV, Ukraine — Russia pressed ahead with its assault on neighboring Ukraine on Thursday, with explosions resounding in cities across the country, airstrikes crippling its defenses and reports of troops crossing the border by land and sea.

An adviser to Ukraine’s interior minister said on Facebook that Russian missiles had struck Ukrainian military command centers, air bases and depots in the capital, Kyiv, and in the major cities of Kharkiv and Dnipro. The government acknowledged that Russian forces had taken control of Chernobyl, the city north of Kyiv that was the site of the world’s worst nuclear accident, raising fears of a possible escape of radioactive material.

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