State Department spokeswoman Jen Psaki said the United States had received reports of heavy weapons being moved into eastern Ukraine from Russia over the past few days, and more apparently on the way.
"This is clearly not in the spirit of this week's agreement," Psaki told reporters.
She said the Russian military had deployed large amounts of artillery and multiple rocket launcher systems and was using it to shell Ukrainian positions.
"We are confident that these are Russian military, not separatist systems," she said, adding that Russian troops along the border were preparing a large shipment of supplies to pro-Russian rebels in eastern Ukraine.
The fragile agreement was seen as the best hope of ending the conflict, which has killed at least 5,480 people and ratcheted East-West tensions to highs not seen since the Cold War, but skepticism remains high after the collapse of a similar previous peace plan.
"All parties must show complete restraint in the run up to the Sunday cease-fire, including an immediate halt to the Russian and separatist assault on the Debaltseve and other Ukrainian towns," Psaki said.