Russia's foreign minister told the world not to underestimate the considerable risks of nuclear conflict, and NATO's supply of weapons to Ukraine "in essence" meant that the Western alliance was engaged in a proxy war with Russia.
Interviewed by Russian state television, Foreign Minister Sergei Lavrov was asked about the importance of avoiding World War Three and whether the current situation was comparable to the 1962 Cuban Missile Crisis.
"The risks now are considerable," Lavrov said according to a transcript of the interview on the ministry's website.
"I would not want to elevate those risks artificially. Many would like that. The danger is serious, real. And we must not underestimate it."
Ukraine's Foreign Minister Dmytro Kuleba said he regarded Russia's scaremongering as a sign of weakness.
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Russia had lost its "last hope to scare the world off supporting Ukraine," Kuleba wrote on Twitter after Lavrov's interview. "This only means Moscow senses defeat."
During a visit to Kyiv on Sunday, U.S. Secretary of State Antony Blinken and Defense Secretary Lloyd Austin promised more military aid for Ukraine.
The U.S. State Department on Monday used an emergency declaration to approve the potential sale of $165 million worth of ammunition to Ukraine. The Pentagon said the package could include artillery ammunition for howitzers, tanks and grenade launchers.
Moscow's ambassador to Washington told the United States to halt shipments, warning Western weapons were inflaming the conflict.
Lavrov said: "NATO, in essence, is engaged in a war with Russia through a proxy and is arming that proxy. War means war."
Russia's two-month-old invasion of Ukraine, the biggest attack on a European state since 1945, has left thousands dead or injured, reduced towns and cities to rubble, and forced over 5 million people to flee abroad.
Moscow calls its actions a "special operation" to disarm Ukraine and protect it from fascists. Ukraine and the West say this is a false pretext for an unprovoked war of aggression by President Vladimir Putin.
The United States is due to host an expected gathering of more than 40 countries this week for Ukraine-related defence talks that will focus on arming Kyiv, U.S. officials said.
Britain said all tariffs on goods coming into the country from Ukraine under an existing free trade deal will be axed and it would send new ambulances, fire engines, medical supplies and funding for health experts to help the emergency services.
Russia's foreign ministry said on Monday that it had declared 40 German diplomatic staff "personae non-gratae" in a retaliatory move after Berlin expelled the same number of Russian diplomats.
WAR RAGES IN SOUTH, EAST
Russia has yet to capture any of the biggest cities. Its forces were forced to pull back from the outskirts of Kyiv in the face of stiff resistance.
"It is obvious that every day - and especially today when the third month of our resistance has begun - that everyone in Ukraine is concerned with peace, about when it will all be over," President Volodymyr Zelenskiy said late on Monday.
"There is no simple answer to that at this time."
Having failed to take the capital Kyiv, Moscow last week launched a massive assault in an attempt to capture eastern provinces known as the Donbas, which if successful would link territory held by pro-Russian separatists in the east with the Crimea region that Moscow annexed in 2014.
Ukraine's general staff said on Tuesday that Russia's offensive continued in the eastern Kharkiv region with Russian forces trying to advance towards Zavody.
Russia's defence ministry earlier said its missiles destroyed six facilities powering the railways that were used to deliver foreign weapons to Ukrainian forces in the Donbas region. Reuters could not verify the report.
The head of Ukraine's state rail company said that one railway worker had been killed and four injured by Russian missile strikes on five Ukrainian railway stations on Monday.
Ukrainian forces have repelled five Russian attacks and killed just over 200 Russian servicemen, said the Ukrainian military command in the southern and eastern sectors.
Five tanks were also destroyed, along with eight armoured vehicles, it said in a statement.
Russia is probably attempting to encircle heavily fortified Ukrainian positions in the country's east, the British military said in an update on Tuesday.
Reports say the city of Kreminna has fallen, with heavy fighting in the south of the city of Izium, as Russian forces try to advance towards the cities of Sloviansk and Kramatorsk, Britain's defence ministry said on Twitter.
The governor of Russia's Belgorod province, Vyacheslav Gladkov, said Ukraine had fired on two villages, and at least two people were wounded. Reuters was unable to verify the Ukrainian or Russian reports.
Russian forces were continuing on Monday to bomb and shell the vast Azovstal steel plant in Mariupol where fighters are hunkered down in a city ravaged by a siege and bombardment, Ukrainian presidential aide Oleksiy Arestovych said.
Moscow said it was opening a humanitarian corridor to let civilians out of the plant but Kyiv said no agreement had been reached.