The invasion of Ukraine is a "dangerous wake-up call for the United Nations", Russian Foreign Minister Sergei Lavrov has claimed.
During a news conference with UN Secretary-General Antonio Guterres in Moscow, Mr Lavrov also alleged the UN was trying to "cross out the basic rules" of its own charter.
The organisation was "created on the basis of sovereign equality of states", he added.
But Mr Guterres said that while he understood Russia has "many grievances", there is "one thing that is true and obvious and that no arguments can change".
He continued: "We have no Ukrainian troops in the territory of the Russian Federation, but we have Russian troops in the territory of the Ukrainian Federation."
He also denied Mr Lavrov’s allegation about the UN charter.
Earlier, Mr Guterres said his priority was to "minimise human suffering" in Ukraine and called for a ceasefire "as soon as possible", while the US said it would "move heaven and earth" to keep supporting Kyiv.
Mr Lavrov said Russia would consider negotiations, adding: "If someone has interesting ideas we are prepared to listen to them."
But it is "too early" to talk about mediators, he said.
During initial comments, Mr Guterres said there were "different interpretations about what is happening in Ukraine".
But that does "not limit the possibility to have a very serious dialogue to minimise the suffering of people", he added.
He also said the UN is "ready to fully mobilise its human and logistical resources to help save lives in Mariupol".
Describing the situation in the city as a "crisis within a crisis", he suggested that the UN and International Committee of the Red Cross – along with Russian and Ukrainian forces – work together to allow people to leave "in any direction they choose".
He said Ukraine "urgently" needs humanitarian corridors that are "truly safe" and "respected by both sides".
Ukraine uses ‘theatrical gestures’, Lavrov claims
During the news conference, however, Mr Lavrov suggested that talks with Ukraine on allowing civilians to leave Mariupol were unlikely to go ahead.
Asked about the prospect of discussions, he said it was a "theatrical gesture" from Ukraine and "they probably wanted another heartrending scene".
"If we talk about serious attitudes to work as part of the talks, they better answer our proposals as soon as possible," he said.
He also claimed Ukrainian President Volodymyr Zelenskyy had been ignoring Russia’s latest proposals.
Russia’s invasion ‘indefensible’
Elsewhere on Tuesday, US Defence Secretary Lloyd Austin said Ukraine’s allies would "keep moving heaven and earth" to aid the country’s security.
He has convened a meeting at America’s Ramstein air base in Germany, with defence ministers and top military leaders from approximately 40 countries participating.
"This gathering reflects the galvanised world," Mr Austin said in his opening remarks, adding that he wanted delegates to leave the meeting with a "common and transparent understanding of Ukraine’s near-term security requirements".
Russia’s invasion is "indefensible" and "so are Russian atrocities", Mr Austin said.
"We believe (Ukraine) can win if they have the right equipment, the right support," he said in Poland on Monday.
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‘War means war’, Lavrov warns
Mr Guterres’s trip to Moscow came hot on the heels of incendiary comments from Sergei Lavrov about a potential escalation of the conflict, claiming Moscow is effectively at war with NATO and that the threat of a nuclear conflict "should not be underestimated".
Weapons supplied by Western countries "will be a legitimate target", Mr Lavrov said, adding: "Everyone is reciting incantations that in no case can we allow World War Three."
Accusing Ukrainian leaders of provoking Russia by asking NATO to become involved in the conflict, he went on: "NATO, in essence, is engaged in a war with Russia through a proxy and is arming that proxy.
"War means war."
But Dmytro Kuleba said Mr Lavrov’s comments showed that "Moscow senses defeat in Ukraine".
He tweeted: "Russia loses last hope to scare the world off supporting Ukraine. Thus the talk of a ‘real’ danger of World War Three."
Germany sends tanks to Ukraine
In what may be a change of heart from German Chancellor Olaf Scholz, a German weapons-maker is expected to be given permission to sell refurbished Gepard anti-aircraft tanks from German Army stock, according to local media.
Mr Scholz had said he was trying to avoid a nuclear war with Russia, although he has come under mounting pressure from international leaders and some in his own country.
Germany’s defence minister also said the country will deliver self-propelled armoured anti-aircraft guns to Ukraine.
Capture of Donbas ‘not inevitable’
Russian forces have now fully withdrawn from around Kyiv and much of northern Ukraine.
The port city of Mariupol is mostly under the control of Russian forces and a large-scale Russian offensive in eastern Ukraine has begun, aiming for the "complete liberation" of the Donbas – the eastern regions of Donetsk and Luhansk.
But UK armed forces minister James Heappey said it is not inevitable that Russia will take parts of the Donbas.
He told Sky News the region will be an "extraordinarily difficult nut for the Russians to crack" and there is "every chance the Ukrainians can see them off".
Lloyd Austin has said the US wants to see Russia "weakened to the point where it can’t do things like invade Ukraine".
On Monday, the US said it will again ramp up the amount of military kit it is sending to Kyiv.
The State Department used an emergency declaration to approve the potential sale of $165m (£129m) worth of ammunition, including artillery ammunition for howitzers, tanks and grenade launchers.
It also said it would provide more than $300m (£235m) in financing to buy more supplies.
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