The accusation, made by the State Department spokeswoman, Jen Psaki, also said that the Russian military had deployed air defense systems near the town of Debaltseve. Thousands of Ukrainian soldiers in Debaltseve have been reported to be surrounded by pro-Russian terrorists and fighting has escalated there ahead of the scheduled Saturday night start of the cease-fire.
"The Russian military has deployed a large amount of artillery and multiple rocket launcher systems around Debaltseve, where it is shelling Ukrainian positions," Ms. Psaki said, reading a prepared statement. "We are confident that these are Russian military, not separatist systems."
Ms. Psaki also said that the United States had reliable reports that Russia was preparing a large shipment of supplies to pro-Russian forces fighting in Ukraine.
This was the first time that the Obama administration has accused Russia of taking a direct hand in an assault on Ukrainian forces since the cease-fire agreement was signed.
The Russians, who have insisted they are not assisting the separatists, helped negotiate the truce agreement reached on Thursday in Minsk, Belarus - the second in the nearly year-old eastern Ukraine war - which calls for a formal halt to hostilities at midnight Saturday.
American officials said that the attack on Debaltseve, a strategically important railroad hub midway between the main militants-held cities of Luhansk and Donetsk, was a violation of the spirit of that agreement.
On Thursday, American officials warned the Kremlin and the separatists not to try to seize new territory before the cease-fire takes effect.