The United States has accused Russia and pro-Russian rebels of violating "the spirit" of a fresh peace deal for eastern Ukraine.
State Department spokeswoman Jen Psaki said Russia had deployed an air defense system at Debaltseve, along with "a large amount of artillery and multiple rocket launcher systems", AP reports.
She added that Washington had intelligence that Russia is preparing to send a large shipment of supplies to the rebels.
Ahead of a cease-fire on February 15, rebels have stepped up attacks on Ukrainian forces in Debaltseve, a town midway between the rebel strongholds of Donetsk and Luhansk.
The violence has dimmed hopes for the peace accord sealed on February 12 in Minsk by the leaders of Russia, Ukraine, Germany and France.
Countries from the Group of Seven (G7) industrialized nations said they were ready to take "appropriate measures" against those who violate the agreement, an apparent threat of more economic sanctions against Russia.