Washington’s move to allow Kyiv to target deep within Russia using long-range US missiles has escalated the Ukraine conflict and could provoke World War Three, senior Russian lawmakers warned on Sunday.

The shift marks a major reversal in US policy, as revealed earlier on Sunday by two US officials and a source familiar with the decision.

“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.