Russia said on Monday that its military was analyzing whether or not the United States would supply Tomahawk cruise missiles to Ukraine for strikes deep into Russia, a step that Russian officials say could trigger a steep escalation.
US Vice President JD Vance said on Sunday that Washington was considering a Ukrainian request to obtain Tomahawks.
President Donald Trump has not made a final decision, and he has been wary of escalating the Ukraine war into a direct confrontation with Russia. But the fact he is now weighing such a move shows the extent of his frustration with President Vladimir Putin's refusal to agree a ceasefire since he hosted the Russian leader at a summit in Alaska last month.
Tomahawks have a range of 2,500 km (1,550 miles) - easily far enough to hit Moscow and most of European Russia if fired from Ukraine.
Ukraine's President Volodymyr Zelenskiy cautioned Kremlin officials last week that they should "know where the bomb shelters are".
RUSSIA CONDUCTING 'IN-DEPTH ANALYSIS' OF SCENARIOS
It was unclear how or through which countries the Tomahawks could be supplied. Zelenskiy has asked Washington to sell them to European nations that would send them to Ukraine.
For the Kremlin, the escalatory risks of US involvement in firing such missiles deep into Russia are clear.
"The question... is this: who can launch these missiles...? Can only Ukrainians launch them, or do American soldiers have to do that?" Kremlin spokesman Dmitry Peskov told reporters when asked about Vance's remarks.
"Who is determining the targeting of these missiles? The American side or the Ukrainians themselves?" Peskov added, saying "a very in-depth analysis" was required.
Putin has previously warned that Russia reserves the right to strike at military installations in countries that let Ukraine use their missiles to hit Russia.
Andrei Kartapolov, head of the Russian parliament's defense committee, told the Mayak news outlet that any US military specialists who helped Ukraine to launch Tomahawks against Russia would become targets for Moscow.
"And no one will protect them. Not Trump, not Kellogg, nor anyone else," he said.
Keith Kellogg, the US special envoy to Ukraine, said on Sunday that Trump had indicated that Kyiv should now be able to conduct long-range strikes on Russia.
"Use the ability to hit deep. There are no such things as sanctuaries," Kellogg told Fox News.
KREMLIN SAYS TOMAHAWKS WON'T HELP 'KYIV REGIME'
However, Kremlin spokesman Peskov on Monday also said any use of Tomahawks would not be a game-changer in the war.
"Even if this happens, there's no panacea that can change the situation on the front for the Kyiv regime right now... And whether it's Tomahawks or other missiles, they won't be able to change the dynamic," Peskov said, referring to the slow but steady gains Russian forces are making in eastern Ukraine.
Former Russian President Dmitry Medvedev said on Monday that Europe "simply cannot afford a war with Russia" but that "the possibility of a fatal accident always exists".
