On Wednesday, Washington said it had approved an export license allowing the sale of light weapons and small arms to Ukraine from commercial U.S. manufacturers.
The U.S. decision to provide lethal weapons brought a predictably sharp rebuke from Moscow, which has tacitly backed the separatists in eastern Ukraine while denying that it actively supports them.
"The United States has crossed a line by announcing its intention to supply lethal weapons to Ukraine," Russian Deputy Foreign Minister Sergei Ryabkov said Saturday. "U.S. weapons are capable of leading to new casualties in our neighboring country, and we cannot remain indifferent to that."
Moscow has long maintained that U.S. authorities organized and oversaw the Maidan protests in Kiev that led then-President Viktor Yanukovych to flee to Russia in early 2014. Russian state television programs constantly portray Ukraine as a failed state with neo-fascist leaders bent on oppressing ethnic Russians in the country's eastern Donbas region.
"The United States is clearly prompting them to a new bloodshed now," Ryabkov said in comments posted on the Russian Foreign Ministry website. "Kyiv's revenge-seekers have already been shelling Donbas every day and are unwilling to hold peace talks, while dreaming about doing away with the disobedient population. And the U.S. has decided to give them the weapons for it."
Moscow also blames deep-seated anti-Russian sentiment for the chill in U.S.-Russian relations.
"Russophobia is obstructing the views of many of them so much now that they cheerfully applaud to punishers from Ukrainian nationalist battalions," Ryabkov said.
Moscow's admonition came shortly after a cease-fire went into effect between Ukraine and Russian-speaking rebels, in acknowledgment of Christmas and New Year's holidays. Almost immediately, each side accused the other of violating the cease-fire.
The conflict in Ukraine began after Russia annexed Crimea in 2014 and then began supporting separatists in the eastern part of the former Soviet country. More than 10,000 people have died in the war, including about 450 in the first 11 months of this year.
Secretary of State Rex Tillerson discussed a recent increase in violence during a phone call Friday with Ukrainian President Petro Poroshenko, according to an account provided by the Ukrainian government. The State Department confirmed that the call had taken place but provided no details.
The Ukrainians said Poroshenko thanked Tillerson for the continuation of U.S. sanctions against Russia and for the "consistent support of Washington regarding the increase of Ukraine's defense capacity." It also said Tillerson reassured him that U.S. support for Ukraine would continue.
Russia has denied sending troops and heavy weaponry to eastern Ukraine, but the United States says Moscow is providing arms, training and troops to support the separatists. Tillerson has said that the strained U.S.-Russia relations cannot improve and that punishing sanctions will not be lifted until Moscow withdraws its support from the rebels in Ukraine and returns Crimea to Ukrainian sovereignty.
Ukraine has for years asked the United States to provide it with lethal weapons. The Obama administration considered providing heavy weapons but balked, fearing such a move would further inflame the conflict. Among the equipment Kiev most seeks are Javelin antitank missiles.
The State Department said the "U.S. assistance is entirely defensive in nature, and as we have always said, Ukraine is a sovereign country and has a right to defend itself."
In Moscow, a leading Russian lawmaker also spoke out against the U.S. arms deal Saturday, saying that it signaled Washington's intention to abandon talks on regulating the Ukraine crisis that were established between U.S. special envoy Kurt Volker and his Russian counterpart, Vladislav Surkov.
"By deciding to arm Ukraine with lethal weapons, the United States is giving a clear signal to Kyiv that it will support an attempt at a military solution," Alexei Pushkov, a member of the defense and security committee of the upper house of the Russian parliament, tweeted on Saturday. "Volker might as well take a vacation."
More-measured voices in the Russian capital also cautioned that the U.S. decision could lead to an escalation of the Ukraine crisis.
"The provision of U.S. lethal arms for Ukraine will change little on the battlefield, but it marks Washington's deeper and more direct involvement in the conflict," Dmitri Trenin, director of the Carnegie Moscow Center, commented on Twitter. "An important line has been crossed, and it probably won't stop there."
This article has been updated to correct the name of the Russian counterpart to the U.S. special envoy on Ukraine, Kurt Volker. It is Vladislav Surkov, not Sergei Ivanov.