Russia on Wednesday sharply criticised a US decision to supply advanced rocket systems and munitions to Ukraine, warning of an increased risk of direct confrontation with Washington.
Kremlin spokesperson Dmitry Peskov told reporters: "We believe that the United States is purposefully and diligently adding fuel to the fire."
When asked how Russia would respond if Ukraine used US-supplied rockets to strike Russian territory, Peskov said: "Let's not talk about worst-case scenarios".
US President Joe Biden has agreed to provide Ukraine with advanced rocket systems that can strike with precision at long-range Russian targets as part of a new US package to help Kyiv defend itself in the three-month-old war that began with Russia's 24 February invasion.
Washington agreed to supply the rockets, which are capable of hitting targets as far away as 80 km (50 miles), after Ukraine gave "assurances" they will not use the missiles to strike inside Russia itself, senior US officials said.
Peskov said Moscow did not trust such assurances. He said it was assessing the risk of rockets being fired into Russian territory and was taking appropriate measures, but that it viewed Washington's step "extremely negatively."
He said such supplies would not encourage Ukraine's leadership to resume stalled peace talks.
Ukrainian officials have been asking allies for longer-range missile systems that can fire a barrage of rockets hundreds of miles away, in the hopes of turning the tide of the war.
US President Joe Biden wrote in an opinion piece in the New York Times: "We have moved quickly to send Ukraine a significant amount of weaponry and ammunition so that it can fight on the battlefield and be in the strongest possible position at the negotiating table."
Earlier, state news agency RIA Novosti quoted Russian Deputy Foreign Minister Sergei Ryabkov as saying, when asked about the prospect of a direct confrontation between the United States and Russia: "Any arms shipments that continue, that are on the rise, increase the risks of such a development."