Russia has warned the United States against sending more arms to Ukraine, Moscow’s ambassador to Washington told Russian state television.

“We stressed the unacceptability of this situation when the United States of America pours weapons into Ukraine, and we demanded an end to this practice,” Anatoly Antonov said in an interview with the Rossiya 24 TV channel.

Antonov said an official diplomatic note had been sent to Washington expressing Russia’s concerns. He said such arms supplies from the United States would further aggravate the situation and raised the stakes of the conflict.

Washington’s top diplomat and its defence secretary met Ukrainian President Volodomyr Zelenskiy in Kyiv late on Sunday, pledging new assistance worth $713 million for Zelenskiy’s government and other countries in the region fearing Russian aggression.

Earlier in April, U.S. President Joe Biden announced an additional $800 million in military assistance to Ukraine, expanding the scope of the systems provided to include heavy artillery.

Zelenskiy has been pleading with U.S. and European leaders to supply Kyiv with heavier arms and equipment. Thousands have been killed and millions displaced since Russia sent troops into Ukraine on Feb. 24 in what it calls a “special military operation” to “demilitarise” its neighbouring country.

Russia’s Feb. 24 invasion of Ukraine has killed thousands of people, displaced millions and raised fears of a wider confrontation between Russia and the United States – by far the world’s two biggest nuclear powers.

President Vladimir Putin says the “special military operation” in Ukraine is necessary because the United States was using Ukraine to threaten Russia and Moscow had to defend against the persecution of Russian-speaking people

Ukraine and the West say Russia began an unprovoked war of aggression.