The United Nations has declared Russia's war on Ukraine "unwinnable", imploring its government to begin peace talks. 

"Sooner or later, it will have to move from the battlefield to the peace table. This is inevitable," said Secretary-General of the United Nations, António Guterres, addressing journalists outside the Security Council. 

"The only question is: How many more lives must be lost? 

"How many more bombs must fall? How many more Mariupols must be destroyed? How many more Ukrainians and Russians will be killed before everyone realises that this war has no winners – only losers?" 

Meanwhile, Ukrainian and US sources have said that Russia's military efforts have been greatly depleted. 

In a press briefing with the US Department of Defence, a senior official places Russia's combat power at "just below 90 per cent". 

They did, however, explain that while they're "expending an awful lot" during combat, they also have "a lot available to them". 

People gather amid the destruction caused after shelling of a shopping centre, in Kyiv, Ukraine, on March 21. Photo / AP 

"We recognise they are taking casualties every day," said the official. 

"They are losing aircraft. They are losing armour and vehicles, no doubt about that. 

"We do see them continue to suffer casualties and losses, but they had – they built up an awful lot of combat power." 

Today, a report from the Ukrainian military claimed Russian forces have a mere three days worth of food, ammunition and fuel left. 

Pentagon officials agreed that Russian forces are "struggling on many fronts," and that several isolated units have reported supply issues, The Guardian reports. 

"It is consistent with an advance which has ground to a halt. Failures in the logistic chain has been one of the reasons they have not been as effective as they hoped," they wrote. 

The Defence Intelligence of Ukraine also claimed that Russia had resorted to "hidden mobilisation" tactics, accusing the Russian Prosecutor's Office of offering to release all credit obligations from people who sign a contract with the army. Similar proposals have also been given to criminals in exchange for full amnesty. 

"Given the total failure to replenish the military reserve, the occupiers are moving to new forms of hidden 'mobilisation,'" a spokesman wrote via their Facebook. 

This satellite image provided by Maxar Technologies shows burning and destroyed apartment buildings in Mariupol, Ukraine, on March 22. Photo / AP 

'Nothing left:' 100,000 trapped in Mariupol 'hellscape' rubble 

Nearly 100,000 people are reported to be trapped inside the ruins of Mariupol. The besieged port city has faced weeks of bombing and shelling from Russian forces, while being cut off from water, electricity, food and medication. 

The conditions are so poor, they've been described by the Human Rights Watch as a "freezing hellscape riddled with dead bodies and destroyed buildings". 

While Ukraine President Volodymyr Zelenskyy estimates 7026 have been able to escape the city, several attempts to create humanitarian corridors and evacuations have previously been sabotaged by Russian attacks. 

On the President's Tuesday nightly address (local time), Zelenskyy said the remaining residents were living in "inhumane conditions," with Russian troops instilling "deliberate terror" on those attempting to flee. 

"Sadly, almost all of our efforts are sabotaged by Russian occupants, by [their] shelling or deliberate terror," he said. 

"Today, one of the humanitarian convoys was seized by occupants on an arranged route near Mangush. 

"Employees of the State Emergency Service and bus drivers have been taken captive. We are doing everything to set our people free and unblocked the movement of humanitarian cargo." 

'Existential threat:' Russia refuses to deny use of nuclear weapons 

In a bold declaration, Vladimir Putin's press secretary refused to quash the potential for nuclear weapons to be used in Russia's assault on Ukraine. 

CNN reports Dmitry Peskov repeatedly denied ruling out the possibility of a nuclear attack in order to achieve Russia's military goals. 

"If it is an existential threat for our country, then it can be," Peskov told CNN's chief international correspondent Christiane Amanpour. 

Peskov doubled down on Russia's goals to "demilitarise" Ukraine and accused the government of being "nationalist battalions". 

When asked about how Russia was progressing in the conflict, Peskov vaguely hinted Putin "hasn't achieved" his goals to date but said it was "going on strictly in accordance with the plans and the purposes that were established beforehand". 

Why Zelenskyy assassination plot failed 

A Russian plot to assassinate Zelenskyy involving a "25-man death squad" has been foiled with the entire group arrested near the Slovakian border, it's been reported. 

However, it's the manner in which the group's actions were uncovered which has raised eyebrows. 

The hitmen were reportedly betrayed by their own side with elements within the Russian secret service, the FSB, informing Kyiv of their location and plans. 

It's thought to be just one in a string of assassination attempts against senior members of the Ukrainian Government which have failed. 

German newspaper Bild reported the group of 25 trained killers, led by a Russian secret service agent, were captured in the western Ukraine city of Uzhhorod on their way to the capital. 

Uzhhorod lies close to the borders of both Slovakia and Hungary. 

As well as an attack on Zelenskyy, the group were also planning "acts of sabotage" in the government district of Kyiv, it was alleged. 

Their plan was to pretend to be members of Ukraine's territorial units to gain access to the city. 

Ukrainian President Volodymyr Zelensky addresses the Italian Parliament via live video from the embattled city of Kyiv. Photo / Getty Images 

But news agency Unian said a faction with the Russian government against the invasion contacted Kyiv informing them of the plot, dooming the mission to failure. 

"There is said to be a group of opponents of the war in the FSB which specifically gave information to the Ukrainians," stated Bild. 

It's thought Moscow has enlisted the help of special forces from the restive Russian region of Chechnya as well as notorious soldiers-for-hire group Wagner to target the Ukrainian presidents. 

In early March it was revealed there were more than a dozen assassination attempts against Zelenskyy in the first two weeks of the war alone. 

"We have a very powerful network of intelligence and counterintelligence – they track it all," presidential adviser Mikhail Podolyak told the outlet Ukrainian Pravda. 

Thwarted attempts by infiltrators have included would-be assassins who have been "liquidated" while attempting to get to Zelenskyy's government quarters, he said.