In a strongly-worded statement, Russian 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.
The move by the Trump administration comes a day after the Kremlin said Russian President Vladimir Putin spoke to German Chancellor Angela Merkel to explain Russia's decision to withdraw its military observers from a joint group monitoring the truce in eastern Ukraine. The stern Moscow warning came after a new ceasefire deal between Ukraine and pro-Russian rebels went into effect from midnight on Saturday ahead of the New Year and Orthodox Christmas holidays.
"The U.S. government is not selling the Ukrainian government these weapons".
Fighting between Ukrainian troops and Russia-backed separatists has killed more than 10,000 since April 2014.
"Earlier this year, the Trump administration approved the sale of shoulder-fired rocket launchers by AirTronic, a US manufacturer in Texas, to Ukraine, according to the company's COO [chief operating officer]", ABC News notes.
The State Department wouldn't confirm that lethal weapons would be sent, saying only that the USA planned to provide "enhanced defensive capabilities".
The Trump administration has not said what kind of weapons will be provided, but media reports have suggested the USA will be giving the Ukrainian military Javelin anti-tank missiles. "The American weapons in the hands of Ukrainian soldiers are not for offensive purposes, but to respond in a resolute way to the aggressor.... and for effective self-defense", said Ukrainian President Petro Poroshenko on Facebook.
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There were 152 ceasefire violations in 2015 and 153 in 2014 on the LoC. Two other personnel also sustained injuries in the firing.
"It is also a transatlantic vaccination against the Russian virus of aggression", Poroshenko added.
Sen. John McCain, an Arizona Republican, released a statement Saturday morning supporting the decision.
Trump had been considering the plan for some time after the State Department and the Pentagon signed off earlier this year.
Under law, the State Department must tell Congress of planned foreign military sales, triggering a review period in which lawmakers can act to stop the sale.
"Russia and its proxies are the source of violence in eastern Ukraine, and the Russian government continues to perpetuate an active conflict and humanitarian crisis through its leadership and supply of military forces on the ground, as well as its direct control over proxy authorities", Nauert told reporters.
Ukraine is to receive more lethal American-made weapons to help it fight Russian-backed separatists.
"It stands as the single most hard obstacle to us renormalizing the relationship with Russian Federation, which we badly would like to do", Tillerson said.