Russia has said that any decision by the United States to allow Ukraine to fire US missiles deep into Russia would mean it was directly involved in the conflict, and accused President Joe Biden's administration of escalating the war.
Russia has been telling the west for months how it would interpret such a decision, and that it would raise the risk of a confrontation with the US-led NATO alliance.
When asked about reports by the New York Times and Reuters that Mr Biden's administration had made the decision on long-range strikes, Kremlin spokesman Dmitry Peskov noted that the reports were not based on any official statement.
"If such a decision was indeed formulated and brought to the Kyiv regime, then this is a qualitatively new round of tension and a qualitatively new situation from the point of view of US involvement in this conflict," Mr Peskov said.
President Vladimir Putin made Russia's position absolutely clear when speaking in St Petersburg in September, Mr Peskov said.
Mr Putin said on 12 September that western approval for such a step would mean "the direct involvement of NATO countries, the United States and European countries in the war in Ukraine" because NATO military infrastructure and personnel would have to be involved in the targeting and firing of the missiles.
"It is obvious that the outgoing administration in Washington intends to take steps to continue adding fuel to the fire and continue to provoke tension around this conflict," Mr Peskov said.
Reuters reported the Biden administration's decision yesterday, citing two US officials and a source familiar with the decision. The New York Times also reported the decision.
Sources quoted in both reports presented the move as partly in response to the reported arrival of North Korean soldiers in Russia's Kursk region to help repel a Ukrainian incursion.
Senior Russian politicians have expressed anger at the reports.
A member of the Russian-parliament, Maria Butina, said the US was risking the start of World War III.
She said she hoped the decision would be reversed by Donald Trump when he succeeded Mr Biden in the New Year.
In late October, Mr Putin said that Russia's defence ministry was working on different ways to respond if the United States and its NATO allies help Ukraine strike deep into Russia with long-range western missiles.
"I guess there are some people in the United States who have nothing to lose for whatever reason or who are completely off the grid so much that they simply do not care," said Ms Butina, who spent 15 months in a US prison for acting as an unregistered Russian agent and is now a politician for the ruling United Russia party.
Ukrainian President Volodymyr Zelensky said the long-range missiles would "speak for themselves".
"Today, many in the media are saying that we have received permission to take appropriate actions. But strikes are not made with words. Such things are not announced. The missiles will speak for themselves," he said in his evening address last night.
Meanwhile, EU foreign policy chief Josep Borrell has expressed his hope that EU members could agree to allow Ukraine to use arms to strike inside Russia.
"I've been saying once and again that Ukraine should be able to use the arms we provided to them, in order to not only stop the arrows but also to be able to hit the archers," Mr Borrell said before a meeting with EU foreign ministers in Brussels.
"I continue believing this is what has to be done. I'm sure we will discuss once again. I hope member states will agree on that."
Separately, North Korean leader Kim Jong-un said the US and the west are using the Ukrainian military as "shock troops" to fight Russia and risk triggering a global conflict, state media reported.
South Korea and the US have accused nuclear-armed North Korea of sending more than 10,000 soldiers to help Russia fight Ukraine, with experts saying Mr Kim was eager for Russia's advanced technology, plus battle experience for his troops, in return.
North Korea has denied the deployment, and Mr Kim did not mention it in a speech to battalion commanders carried by the official Korean Central News Agency.
The US and the west are using the conflict in Ukraine to "expand the scope of their military interventions globally", Mr Kim said.
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They are also trying to "enhance their combat experience, with Ukraine being used as shock troops" against Russia, he said.
Washington's "continuing military assistance to Ukraine... raises the concern of World War III," he said.
Mr Kim vowed his country would bolster its nuclear weapons defence "without limit".
His warning comes after South Korea said last week that North Korean troops had already begun "engaging in combat operations" alongside Russian forces near the border with Ukraine.
Mr Kim "is likely keeping in mind the possibility of additional deployments to support Russia's war in Ukraine," said Hong Min, a senior analyst at the Korea Institute for National Unification.
Last week, North Korea ratified a landmark defence pact with Russia, formalising months of tightening military bonds between two nations that were Communist allies throughout the Cold War.
In exchange for sending troops, the west fears Russia is offering North Korea technological support that could advance Pyongyang's nuclear weapons programme.
The reclusive state recently fired a salvo of ballistic missiles and tested a new solid-fuel ICBM.
The nuclear-armed state's deployment of troops to Russia has led to a shift in tone from South Korea, which has resisted calls to send lethal weapons to Kyiv so far but recently indicated it might change its no-provision policy.
