Russia has warned the United States against sending more weapons to Ukraine, saying the delivery of arms supplies is “pouring oil on the flames”.

The US has provided about $6.4bn in military assistance to Ukraine since 2014, with $3.7bn of that allotted in a series of packages announced in the wake of Russia’s invasion of its neighbouring country some two months ago.

In comments made on the Rossiya 24 TV channel, Anatoly Antonov, Moscow’s ambassador to the US, said the delivery of military equipment to Ukraine raised the stakes of the conflict, now in its 61st day.

“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,” Antonov said, referring to an official diplomatic note sent to Washington expressing Moscow’s concerns.

“What the Americans are doing is pouring oil on the flames,” Antonov added. “I see only an attempt to raise the stakes, to aggravate the situation, to see more losses.”

US officials in Kyiv

US Secretary of State Antony Blinken and Secretary of Defense Lloyd Austin visited Ukraine’s capital, Kyiv, late on Sunday and met President Volodymyr Zelenskyy, the first visit by US officials since the war began on February 24.

They pledged new assistance worth $713m for Zelenskyy’s government and other countries in the region, and promised US envoys would return to Ukraine soon. On Monday, President Joe Biden nominated Bridget Brink, the current US ambassador to Slovakia, to be Washington’s envoy to Ukraine, stepping up diplomatic measures as well.

“In terms of Russia’s war aims, Russia has already failed and Ukraine has already succeeded,” Blinken told reporters in Poland after the two officials returned from Kyiv, which ran three hours instead of an allotted 90 minutes.

Zelenskyy has been pleading with US and European leaders to supply Kyiv with heavier arms and equipment.

Last week, the US announced its latest $800m military aid package for Ukraine, expanding the scope of the systems provided to include heavy artillery as Ukrainian forces try to stave off a major offensive in their country’s east.

“They [the Ukrainians] need long-range fires. You’ve heard them express the need for tanks and we are doing everything that we can to get them the types of support, the types of artillery and munitions that will be effective in this stage of the fight,” said Austin.

“We want to see Ukraine remain a sovereign country, a democratic country, able to protect its sovereign territory,” he added. “We want to see Russia weakened to the degree that it can’t do the kinds of things that it has done in invading Ukraine. So it has already lost a lot of military capability and a lot of, a lot of its troops, quite frankly.”

Meanwhile, the European Union is reportedly preparing “smart sanctions” against Russian oil imports, possibly an oil embargo, the Times newspaper said on Monday, citing the European Commission’s executive vice president, Valdis Dombrovskis.

An estimated thousands of people have been killed, if not more, and millions displaced since Russia sent troops into Ukraine in what it calls a “special military operation” to “demilitarise” and “denazify” its neighbouring country.

The invasion has raised fears of a wider confrontation between Russia and the US – by far the world’s two biggest nuclear powers.

Meanwhile, Ukraine reported more Russian offensives to take full control of the Donbas – the eastern Donetsk and Luhansk regions, partially held by Russian-backed separatists since 2014 – and to link them and Crimea, which Moscow annexed in that year.

Russia has also been shelling Ukraine’s second-biggest city, Kharkiv, in the northeastern part of the country, and towns and villages to the south.