Moscow accused Nato of engaging in a proxy battle against Russia by arming Ukraine, saying this had created a serious and real risk of nuclear war.

In a marked escalation of Russian rhetoric, Foreign Minister Sergei Lavrov was asked on state television 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 the ministry's transcript of the interview.

“I would not want to elevate those risks artificially. Many would like that. The danger is serious, real. And we must not underestimate it,” Lavrov said.

“Nato, in essence, is engaged in a war with Russia through a proxy and is arming that proxy. War means war.”

Lavrov's remarks came as US Secretary of Defence Lloyd Austin was set to convene a meeting of more than 40 countries at a German air base to discuss arming Ukraine to help it fight off Russia's latest military assault in the east.

“The next several weeks will be very, very critical,” Chairman of the US Joint Chiefs of Staff Mark Milley told reporters travelling with him. “They need continued support in order to be successful on the battlefield. And that's really the purpose of this conference.”

The aim is to coordinate aid that includes heavy weapons such as howitzer artillery, as well as killer drones and ammunition, General Milley said.

Ukraine's Foreign Minister Dmytro Kuleba said he regarded Russia's scaremongering as a sign of weakness.

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.”

Britain also played down the Russian threat.

“Lavrov's trademark over the course of 15 years or so that he has been the Russian foreign secretary has been that sort of bravado. I don't think that right now there is an imminent threat of escalation,” Armed Services Minister James Heappey told BBC Television.

The US State Department on Monday approved the potential sale of $165 million worth of ammunition to Ukraine. The Pentagon said the package could include 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.

Read: Is Russia as isolated as Ukraine’s allies hope?

Guterres to Moscow

US Secretary General Antonio Guterres was due in Moscow on Tuesday to meet President Vladimir Putin and Lavrov, the highest profile peace mission since the war began, although Western countries have said they have little hope of a breakthrough.

Russia's two-month-old invasion of Ukraine has left thousands dead or injured, reduced towns and cities to rubble, and forced more than five million people to flee abroad.

Moscow calls its actions a “special operation” to disarm Ukraine and protect it from fascists. Ukraine and the West call this a false pretext for an unprovoked war of aggression.

War rages in South, East

Russia has yet to capture any of Ukraine's biggest cities. Its huge invasion force was forced to pull back from the outskirts of Kyiv in the face of stiff resistance last month. But it has since announced new war aims to focus mainly on the east, and sent more troops there for an assault on two provinces where it has backed a separatist revolt.

“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.”

Ukraine's general staff said on Tuesday that Russia's offensive continued in the eastern Kharkiv region with Russian forces trying to advance towards a village called Zavody.

While focusing on the east and south, Russia has also been striking targets in other parts of Ukraine with missiles and air strikes. Its defence ministry said it had destroyed six facilities powering railways used to deliver foreign weapons to Ukrainian forces.

The head of Ukraine's state rail company said one railway worker had been killed and four injured by Russian missile strikes on five 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. The reports could not be verified.

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 town of Kreminna has fallen, with heavy fighting in the south of the city of Izyum, 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 border province, Vyacheslav Gladkov, said Ukraine had fired on two villages across the frontier and at least two people were wounded.

560 fighters killed

Russia pummelled Ukraine with missiles, aircraft and artillery overnight, killing at least 560 Ukrainian fighters and destroying dozens of armoured vehicles, rocket systems and other military equipment, the defence ministry said.

Russian aircraft struck 87 different military installations while rockets and artillery rained down on Ukrainian positions, destroying S-300 missile systems, a Tochka U short-range ballistic missile system, BUK-M1 and Osa-AKM missile systems.

“About 500 enemy personnel, 59 armoured vehicles, artillery guns and cars were destroyed, as well as more than 60 militants of the nationalist 'Donbas' group in the Donetsk People's Republic,” the defence ministry said.