Russia warned the U.S. Thursday not to provide arms to Ukrainian government forces fighting pro-Russian rebels in the east of the country, after a U.S. official suggested that the Obama administration was considering supplying lethal military assistance.

Russian Foreign Ministry Spokesman Alexander Lukashevich said that the suggestion that the U.S. would provide lethal military assistance to Ukraine sent a “very serious signal,” adding that it would constitute “a major change in policy of the [U.S.] administration in regard to the conflict” in Ukraine.

“That would be a direct violation of agreements reached, including [those] with the participation of the United States,” Lukashevich said, according to a report in the Guardian

U.S. Deputy National Security Adviser Tony Blinken told a Senate committee Wednesday that the U.S. providing lethal military equipment was “something we should be looking at” to get Russia to think twice about its destabilizing behavior.

Blinken also cited “serious” violations by Russia of agreements not to interfere in its neighbor's civil strife, according to a report from CBS News

The spat emerged just as U.S. vice president Joe Biden arrived in Kiev for talks with the country's president Petro Poroshenko. In addition to discussing future U.S. economic and military support for the country's elected government, the central message of Biden's visit was to tell Ukraine “we've got your back,” according to a senior White House official quoted by Voice of America

The U.S. has been providing Ukraine with non-lethal military assistance, including Humvee vehicles and mortar detection systems. Biden is expected to announce an increase in the amount of non-lethal military aid during his visit to Kiev, according to Reuters.

NATO leaders have been highly critical of Russia's actions surrounding the crisis in Ukraine. On Nov. 18, NATO Secretary General Jens Stoltenberg criticized the country amid evidence of a “very serious” military buildup in eastern Ukraine and on the Russian side of the border. 

A NATO commander also said that Russian troops, tanks artillery and air defense equipment had been seen crossing the Ukrainian border earlier this month. 

Ukraine has been fighting pro-Russian separatists in the east of the country for most of 2014. Russia supports the separatists, but has denied that its forces have been directly involved in the fighting, a claim contested by both Ukraine and Western powers.