The United States has called on Russia to "stop fueling the fire" in eastern Ukraine with new weapons and other support for pro-Russian separatists there.
U.S. State Department spokeswoman Jen Psaki said Washington condemned "Russia's increased militarization of the Donbas region through the provision of tanks and other heavy equipment to separatists."
Her comments in Washington on November 10 come days after the Organization for Security and Cooperation in Europe (OSCE) said its monitors had witnessed an apparent buildup of heavy weapons and tanks in parts of the Donetsk region controlled by the separatists.
Psaki accused Russia and the separatists of "blatant violations" of a cease-fire signed in Minsk on September 5, and warned that the costs to Moscow will rise if it "continues its destabilizing and dangerous actions."
Psaki said Moscow must do more if it truly wants peace in eastern Ukraine.
"If Russia is truly committed to Minsk and peace in Ukraine, it will stop fueling the fire with new weapons and support for separatists and withdraw all Russian military personnel and equipment from Ukraine; and it will call on its proxies to stop cease-fire violations, release hostages, and close the international [Russian-Ukrainian] border," Psaki said.
Fears About Cease-Fire
The United States and European Union have imposed economic sanctions on Moscow since Russia seized Ukraine's Crimean Peninsula in March and began backing pro-Russian rebels who rose up in the two eastern regions of Donetsk and Luhansk.
U.S. President Barack Obama and Russian President Vladimir Putin had several brief encounters during a November 10-11 Asia-Pacific summit in Beijing, but it was unclear whether they discussed the situation in Ukraine in any detail.
Some of their encounters appeared chilly.
The past week has seen the truce deteriorate, after the rebels staged elections and inaugurated leaders, steps Kyiv and the West say violate the Minsk cease-fire agreement.
The OSCE confirmed on November 8 that a column of troops and tanks without insignia had been spotted in rebel-held territory.
Moscow denies sending troops or arms into Ukraine.
But Psaki noted the Kremlin has declared the circumstances of deaths of Russian soldiers allegedly killed in Ukraine a state secret.
"We know that the families of those killed in action may never have the comfort of knowing from their own government what truly happened to their sons now that their fate has been declared a state secret," Psaki said.
In a blog posting earlier on November 10, a newspaper publisher and local lawmaker in the Russian city of Pskov said Russian military prosecutors had refused to provide details on the circumstances of the deaths of 12 servicemen who died away from their posts earlier this year.
Lev Shlosberg has gathered evidence suggesting Russian soldiers have died in eastern Ukraine.
He was badly beaten by unidentified assailants after his newspaper published an investigation into the funerals of two paratroopers in August.