Washington (AFP) - The White House said that Russia's growing military presence in Ukraine marked a "significant escalation" of the conflict.
"Repeated Russian incursions into Ukraine unacceptable. Dangerous and inflammatory," National Security Adviser Susan Rice said on Twitter.
Her comments came after Kiev's security service said its army in the war-torn east had captured 10 Russian paratroopers on Ukrainian soil, ramping up tensions on the eve of crunch talks on the conflict.
And on Tuesday, Ukrainian President Petro Poroshenko is due to meet Russian leader Vladimir Putin for the first time in months alongside top EU officials in Minsk.
Rice pointed to Russian "military incursions" into Ukraine, listing artillery, air defense systems, dozens of tanks and military personnel.
"Russia has no right to send vehicles or cargo into Ukraine without Govt of Ukraine's permission," she said.
"Russia's military incursions into Ukraine- artillery, air def systems, dozens of tanks & military personnel–represent significant escalation."
Kiev has long accused Moscow of stoking the separatist insurgency raging in its east but this was the first time Ukrainian authorities have claimed to take soldiers from Russia's regular army captive.
The Ukrainian military also halted a column of tanks and armored personal carriers flying rebel flags from Russia heading toward the government-held port city of Mariupol.
Moscow has repeatedly denied any involvement in the rebellion in Ukraine.