Ukraine on Monday welcomed a decision by the Biden administration to allow long-range strikes inside Russia with American-provided missiles, with President Volodymyr Zelensky suggesting that the first launches would come soon and without warning.

Ukraine has long argued that firing at targets deeper inside Russia would allow its military to fight with hands untied, and has positioned it as a just response to years of Russian bombardments.

Ukraine is now facing a combined Russian and North Korean offensive against its troops in territory it captured in the Kursk region of southern Russia over the summer, and the addition of troops from Pyongyang appeared to be the development that persuaded the White House to shift its stance on long-range missiles.

The North Korean deployment alarmed the United States and European nations, as a widening of the war and by drawing Russian allies directly into the ground combat.

Mr. Zelensky, speaking in his nightly address to the nation on Sunday, suggested there would be no warning of the first launches.

“Blows are not inflicted with words,” he said. “Such things are not announced. The rockets will speak for themselves.”

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