Russia has warned that relations with the U.S. are on the “verge of collapse” days after President Joe Biden referred to Vladimir Putin as a “war criminal.”

On Monday, the Russian Foreign Ministry summoned U.S. Ambassador to Russia John Sullivan, an appointee of former President Donald Trump, to hand him a formal letter that called Biden's comments "unacceptable" and warned that relations were near a breaking point.

“It is emphasized that such statements by the American President, unworthy of a statesman of such a high rank, put Russian-American relations on the verge of breaking," the Russian ministry said in a statement.

The Russian protest came in response to Biden remarking to a reporter last week that he considered Putin a war criminal for initiating his attack on Ukraine. Pointing to reports of Russian military strikes against Ukrainian civilians, Biden described them as "atrocities" and promised that Putin would pay a "very heavy price" for these actions.

Biden initially declined to refer to Putin as a war criminal, a legal designation that follows a certain set of criteria, but responded in the affirmative that he personally viewed him as one when pressed by a reporter.

“I think he is a war criminal,” Biden said after clarifying the reporter’s initial question.

Relations between the United States and Russia have been on a road of steady deterioration for the better part of the last decade.

After Russia illegally annexed the Crimean Peninsula from Ukraine in February 2014, the U.S. and the European Union imposed sanctions on Moscow for the move. Relations only worsened after Russians hacked and leaked documents from the Democratic National Committee in the 2016 presidential election in an operation that the CIA assessed was done in part to bolster the candidacy of Trump.

Despite Trump’s own personal affinity for Putin, relations between the U.S. continued to decline. Trump’s cabinet was generally hawkish when it came to Russia and he often bristled at the harsh measures they and members of Congress pushed through to his desk.

In contrast, Biden arrived in office with years of experience in dealings with Putin and promised he would pursue a “stable, predictable” relationship with Russia. Those efforts were stymied after the Biden administration warned for months of Russia’s plans to invade and promised severe consequences.

Ukraine President Volodomyr Zelensky has persistently pressed Biden to do more to respond to Russia’s invasion. The Biden administration responded with billions in military aid sent to Ukraine, but it has backed away from what it sees as riskier measures such as the implementation of a no-fly zone or by greenlighting the transfer of Polish MIG-29s to the Ukrainian air force.

Biden has defended his reticence to increase military support to Ukraine by consistently warning that any clash between the U.S. and Russia, both nuclear powers, would mean "World War III."