Russia slammed the US decision to provide Ukraine with "enhanced defensive capabilities", saying it could lead to "new bloodshed" in the eastern part of the war torn country.
Washington announced that president Donald Trump's administration approved a plan to provide lethal 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.
In a strongly-worded statement deputy foreign minister Sergei Ryabkov accused the United States of "crossing the line" and fomenting the conflict in eastern Ukraine, a region known locally as Donbass.
"Kiev revanchists are shooting at Donbass every day, they don't want to conduct peace negotiations and dream of doing away with the disobedient population. And the United States has decided to give them weapons to do that."
"Today the United States clearly pushes [Ukrainian authorities] towards new bloodshed," Mr Ryabkov said.
"American weapons can lead to new victims in our neighbour," he said.
Another deputy foreign minister, Grigory Karasin, said the US move would undermine efforts to reach a political settlement for Ukraine.
"Essentially this decision undermines the work to implement the 2015 Minsk agreements," Mr Karasin told Tass state news agency, referring to a Western-brokered peace deal.
Previously, Washington has provided Ukraine with support equipment and training, and has let private companies sell some small arms like rifles.
The 2015 peace deal brokered by France and Germany has helped reduce the scale of fighting in eastern Ukraine, but clashes have continued and the agreement's provisions for political settlement have stalled.
The move is likely to become another sore point between Washington and Moscow, as Mr Trump contends with ongoing questions about whether he's too hesitant to confront the Kremlin.
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Meanwhile, German chancellor Angela Merkel and French president Emmanuel Macron urged on Saturday both sides in eastern Ukraine to live up to cease-fire agreements amid a recent upswing in clashes between Russia-backed separatists and Ukrainian forces.
Ms Merkel and Mr Macron issued a joint statement urging combatants to observe the Minsk agreements aimed at defusing 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.
The two leaders also urged the return of Russian military officers to a joint coordination center that plays a role in supporting observers from the Organization for Security and Cooperation in Europe.
They said that "there is no alternative to an exclusively peaceful solution to the conflict".
