President Volodymyr Zelensky has said that Ukraine’s surprise offensive into western Russia, which entered its third week on Tuesday, shows the West that its fears about attacks on Russian territory are unfounded and should be abandoned.

As his forces attempted to push deeper into Russian territory, Mr. Zelensky seized the moment to challenge a limitation from Kyiv’s allies that has long frustrated Ukraine: the use of Western-supplied long-range weapons against Russia, which Ukraine argues is key to disrupting Moscow’s military operations.

“The whole naïve, illusory concept of so-called red lines regarding Russia, which dominated the assessment of the war by some partners, has crumbled these days somewhere near Sudzha,” Mr. Zelensky told Ukrainian ambassadors to other countries in a speech published on Monday evening. He was referring to the western Russian town of Sudzha, which Ukrainian forces captured last week.

For more than two years, Washington has prevented Ukraine from using the weapons it supplied to strike into Russia, citing fears of an escalating conflict between Moscow and the West. This spring, after months of Ukrainian lobbying, the United States and other NATO countries adjusted their policies and granted permission for Ukraine to do that.

But the Biden administration said that Ukraine could only use American weapons to strike military targets a short distance into Russia.