Moscow warned the West Friday against sending troops to Ukraine to buttress its ally, after Kiev accused Moscow of building up troops on its border.
Ukrainian President Volodymyr Zelensky on Thursday accused Russia of massing troops on the border and Washington pledged to stand by Ukraine in the event of Russian "aggression".
Weeks of renewed frontline clashes have raised fears of a an escalation of the long-simmering conflict in eastern Ukraine, where Kiev's forces are battling pro-Russian separatists.
Speaking to reporters on Friday, Kremlin spokesman Dmitry Peskov said Russia would be forced to respond if the US sent troops to Ukraine.
"There is no doubt such a scenario would lead to a further increase in tensions close to Russia's borders. Of course, this would call for additional measures from the Russian side to ensure its security," Peskov told reporters.
He declined to specify which measures would be adopted, while insisting that Russia was not making moves to threaten Ukraine.
"Russia is not threatening anyone, it has never threatened anyone," Peskov said.
His comments come after the United States warned Russia against "intimidating" Ukraine, with both Defence Secretary Lloyd Austin and Secretary of State Antony Blinken calling their Ukrainian counterparts to stress support.
The Pentagon said earlier this week that US forces in Europe had raised their alert status following the "recent escalations of Russian aggression in eastern Ukraine".
Increase in fighting
Ukraine has been battling pro-Russian separatists in the eastern Donetsk and Lugansk regions since 2014, following Moscow's annexation of the Crimean peninsula after an uprising that ousted Ukraine's Kremlin-friendly president Viktor Yanukovych.
Moscow and Kiev this week blamed each other for a rise in violence along the frontline that has undermined a ceasefire brokered last year.
Zelensky said Thursday that 20 Ukrainian servicemen had been killed since the start of the year.
Ukraine's military intelligence accused Russia of preparing to "expand its military presence" in the separatist-controlled regions.
Moscow has repeatedly denied sending troops and arms to support the separatists and the Kremlin this week said that Russia is at liberty to move troops on its own territory.
"Russia is not a participant of the conflict," Peskov said Friday, accusing Ukraine's armed forces of "multiple" provocations in the region.
A senior Russian official dismissed reports of Russia planning an attack on Ukraine as "fake".
"Russia is not interested in any conflict with Ukraine, especially a military one," deputy foreign minister Andrei Rudenko told state news agency RIA Novosti.
Zelensky was elected in 2019 on promises of ending the conflict, but critics say a shaky ceasefire was his only tangible achievement.
The fighting has claimed more than 13,000 lives since 2014, according to the United Nations.