Senior Russian lawmakers said Sunday that any Ukrainian strike on Russia using US missiles could lead to World War Three, Reuters reported.

Washington's decision to let Kyiv strike deep into Russia with long-range US missiles has escalated the conflict in Ukraine. Two US officials and a source familiar with the decision revealed the significant reversal of Washington's policy in the Ukraine-Russia conflict earlier on Sunday.

"The West has decided on such a level of escalation that it could end with the Ukrainian statehood in complete ruins by morning," Andrei Klishas, a senior member of the Federation Council, Russia's upper chamber of parliament, said on the Telegram messaging app.

Vladimir Dzhabarov, first deputy head of the Russian upper house's international affairs committee, said that Moscow's response will be immediate.

"This is a huge step towards the start of World War Three," the TASS state news agency quoted Dzhabarov as saying.

President Vladimir Putin said in September that the West would be fighting Russia directly if it allowed Ukraine to strike Russian territory with Western-made long-range missiles, a move he said would alter the nature and scope of the conflict.

Russia would be forced to take what Putin called "appropriate decisions" based on the new threats.

Leonid Slutsky, chairman of the State Duma lower house's foreign affairs committee, said that US authorisation of strikes by Kyiv on Russia with US ATACMS tactical missiles would lead to the toughest response, Russian news agencies reported.

"Strikes with US missiles deep into Russian regions will inevitably entail a serious escalation, which threatens to lead to much more serious consequences," TASS news agency quoted Slutsky as saying.