Russia's Foreign Ministry says if the United States decides to supply Kyiv with longer-range missiles, it would cross a "red line" and become "a party to the conflict".
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In a briefing, Foreign Ministry spokeswoman Maria Zakharova added that Russia "reserves the right to defend its territory".
Washington has supplied Ukraine with advanced rockets that can hit targets up to 80km away, while so far holding back from publicly announcing it would send rockets with more than double that range.
US officials say Ukraine has promised not to use US rockets to strike Russian territory.
"If Washington decides to supply longer-range missiles to Kyiv, then it will be crossing a red line, and will become a direct party to the conflict," Zakharova said.
Ukraine has requested and received large quantities of weapons from the United States and other Western allies to help it resist the Russian armed forces that were sent into Ukraine in February.
Moscow says it sent troops to prevent Ukraine being used as a platform for Western aggression and to defend Russian speakers.
Kyiv and its Western allies dismiss these arguments as baseless pretexts for an imperial-style war of aggression.
Australian Associated Press
