Washington has "neither encouraged nor enabled" Ukraine to strike targets inside Russia, US Secretary of State Antony Blinken has said.
He was speaking shortly after Moscow accused Kyiv of carrying out drone attacks on three Russian airfields, two of them hundreds of miles from Ukraine.
Ukraine has not commented on the issue.
Russian President Vladimir Putin has repeatedly warned the US and its allies not to cross "red lines" by supplying long-range weapons to Ukraine.
The US-led Nato military alliance has ruled out providing such arms to Kyiv, amid concerns that this could lead to a major escalation with a nuclear-armed Russia that invaded Ukraine on 24 February.
Two Russian airfield explosions were reported on Monday, in the Ryazan and Saratov regions. The sites house strategic bombers used to carry out regular missile attacks on Ukraine's critical energy infrastructure.
Russia said there was another attack on Tuesday, in the Kursk region that borders Ukraine.
Russia's latest missile attack on Ukraine was on Monday, when 70 rockets were fired on targets across the country. Four people were killed, local officials said.
Millions of people across the country are now without electricity and running water, raising fears people may die of hypothermia in sub-zero temperatures.
At a briefing on Tuesday, Mr Blinken accused Russia of "trying to take out the civilian infrastructure that is allowing people to have heat, to have water, to have electricity".
He said Moscow was now "weaponising winter" and "that is the daily and nightly reality in Ukraine".
"We have neither encouraged nor enabled the Ukrainians to strike inside of Russia, but the important thing is to understand what Ukrainians are living through every day with the ongoing Russian aggression against their country."
He said he was determined that Ukrainians have "the equipment that they need to defend themselves, to defend their territory, to defend their freedom."
Speaking alongside him, US Defence Secretary Lloyd Austin stressed that the US would not prevent Ukraine from developing its own long-range strike capability.
"The short answer is no. We are absolutely not doing that," Mr Austin said, adding that Washington had already given Ukraine more than $19bn (£16bn) in security assistance.
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