The United States on Monday accused Moscow of providing pro-Russian separatists in Ukraine with military equipment such as tanks and rocket launchers.
Washington also claimed Moscow was redeploying forces to the volatile border between the ex-Soviet states.
"While Russia says it seeks peace, its actions do not match its rhetoric. We have no evidence that Russia's support for the separatists has ceased," the State Department said.
"In fact, we assess that Russia continues to provide them with heavy weapons, other military equipment and financing, and continues to allow militants to enter Ukraine freely."
The allegations came as Ukrainian forces hunted for the crew of a downed military plane after Kiev said the aircraft was "likely" shot down from Russia, ratcheting up tensions between the two countries.
In a detailed "fact sheet," the State Department singled out a "deployment site" in southwest Russia, saying Moscow "continues to accumulate significant amounts of equipment" there and had roughly doubled the number of tanks, armored vehicles, and rocket launchers at the location.
Washington noted that "more advanced defense systems" have arrived at the site, which also houses artillery, and that it was confident Moscow was mobilizing additional tanks that are no longer in the army's active inventory to the spot.
"We are concerned much of this equipment will be transferred to separatists, as we are confident Russia has already delivered tanks and multiple rocket launchers to them from this site," it said.
"Available information indicates Moscow has recently transferred some Soviet-era tanks and artillery to the separatists and that over the weekend several military vehicles crossed the border."
The State Department accused Moscow of redeploying new forces "extremely close to the Ukrainian border," adding Washington has "information that a significant number of additional military units are also in the process of deploying to the border."
Washington said recruiting efforts for separatist militants were growing inside Russia and that the "Donetsk Peoples' Republic" had set up an office for this purpose in Moscow.
"As the United States and our European allies have repeatedly stated, we call on the Russian government to halt its material support for the separatists, to use its influence with the separatists to push them to lay down their arms and abide by a ceasefire and to release all hostages," it said.