Kremlin spokesman Dmitry Peskov told reporters Moscow suspected a U.S. decision to let Kyiv fire such missiles into Russia had already been taken and that there would be a response if that happened.

"It (our response) will be an appropriate one," said Peskov.

"The involvement of the United States of America and European countries in the conflict over Ukraine is direct, and each new step increases the degree of this involvement," he said.

Vyacheslav Volodin, the chairman of the State Duma, the lower house of parliament, and a close ally of President Vladimir Putin, said Moscow would be forced to use "more powerful and destructive weapons" against Ukraine if Kyiv started firing long-range Western missiles at Russia.

"Washington and other European states are becoming parties to the war in Ukraine," Volodin said on Telegram.