The angry response from Moscow comes a day after President Donald Trump's administration approved a plan to provide weapons to Ukraine, including Javelin anti-tank missiles. Ukraine has long sought the weapons for its fight against Russia-backed separatists in eastern Ukraine that has killed more than 10,000 since April 2014, AP reports.
Russian Deputy Foreign Minister Sergei Ryabkov said that the U.S. administration's move has "crossed a line," adding that the U.S. can no longer claim a mediatory role.
"Washington has sought to cast itself as a 'mediator,'" he said in a statement. "It's not a mediator. It's an accomplice in fueling a war."
Without mentioning the U.S. decision, France and Germany on Saturday urged combatants to fully implement a much-violated cease-fire agreement.
German Chancellor Angela Merkel and French President Emmanuel Macron issued a joint statement urging combatants to observe the 2015 Minsk agreements aimed at de-escalating the conflict. Those provisions include the withdrawal of heavy weapons such as tanks and rocket launchers from the front-line area and an exchange of prisoners.
Read alsoU.S. says it will provide Ukraine with 'defensive' aid – ReutersThe two leaders also urged the return of Russian military officers to a joint coordination center that plays a role in monitoring the cease-fire.
Merkel and Macron said in their statement that "there is no alternative to an exclusively peaceful solution to the conflict."
The U.S. and its allies say Russia has sent sending troops and weapons to help the rebels in eastern Ukraine. Moscow has denied the accusations but acknowledged that Russian citizens joined the separatist forces as volunteers.
Deputy Foreign Minister Ryabkov warned that the U.S. move could warrant a Russian response.
"The American weapons can lead to more victims in the neighboring country, and we couldn't stay indifferent to that," he said.
Valentina Matviyenko, the speaker of the upper house of Russian parliament, said in remarks carried by Tass that the U.S. move was a "big mistake" that would "pull them into Ukraine's internal conflict."
"With lethal weapons supplies, the U.S. gives a clear signal to Kyiv that it will support a military option," Alexei Pushkov, the head of the upper house's information committee, said on Twitter.
Tensions in the east have increased in recent weeks, with observers from the Organization for Security and Cooperation in Europe expressing concern about a recent spike in fighting,
Earlier this week, Russia withdrew its military observers from the joint group monitoring the truce, citing "restrictions and provocations" by Ukrainian authorities that made it hard for Russian officers to perform their duties.
Ukraine and the rebels have declared an intention to speed up efforts to exchange prisoners and maintain a cease-fire for the Christmas and New Year's season starting Saturday.