Ukrainian President Volodymyr Zelensky has said it's "time to talk" with Vladimir Putin and warned it would lead to losses that would take "generations to recover" if he doesn't.
Russia has so far failed to make the military breakthroughs that was widely expected when it launched its Ukraine invasion on February 24.
It has since raised the question whether it would be capable of an all out victory despite its considerably larger military.
Zelensky has said Ukraine had always offered solutions for peace and wanted meaningful and honest negotiations on peace and security, without delay.
His latest appeal to Putin comes with Ukrainian cities now having suffered weeks of heavy shelling leading to mass civilian casualties, and Russia has said it is "tightening the noose" on the port city of Mariupol which has been reduced to rubble.
Zelensky gave a video address in the early hours of Saturday saying: "I want everyone to hear me now, especially in Moscow. The time has come for a meeting, it is time to talk.
"The time has come to restore territorial integrity and justice for Ukraine. Otherwise, Russia's losses will be such that it will take you several generations to recover."
The two sides have been involved in talks for weeks with no sign of a breakthrough.
Zelensky said Russian forces were deliberately blocking the supply of humanitarian supplies to cities under attack.
"This is a deliberate tactic ... This is a war crime and they will answer for it, 100%," he said.
The Ukrainian president said there was no information about how many people had died after a theatre in the city of Mariupol, where hundreds of people had been sheltering, was struck on Wednesday.
More than 130 people had been rescued so far, he said.
It comes as Britain's chief of defence intelligence has warned that Putin is now engaged in a deadly war of attrition as Russia's advance in Ukraine continues to stall in the face of fierce resistance.
Lieutenant General Sir Jim Hockenhull said the Kremlin has been forced to switch tactics, turning to the "reckless and indiscriminate" use of firepower which will inevitably lead to more civilian casualties.
The Russians have "enormous" stocks of artillery ammunition and could maintain their bombardment for weeks in an attempt to force Ukraine into submission, Western officials say.
The warning came as Putin made a rare public appearance to address a mass flag-waving rally at Moscow's Luzhniki Stadium to mark the eighth anniversary of his country's annexation of Crimea.
He praised the efforts of Russia's forces, declaring: "Shoulder to shoulder, they help and support each other."
But Gen Hockenhull said that, more than three weeks into the campaign, it is clear the Kremlin has still not achieved any of its initial objectives.
"It has been surprised by the scale and ferocity of Ukrainian resistance and has been bedevilled by problems of its own making," he said.
"Russian operations have changed. Russia is now pursuing strategy of attrition. This will involve the reckless and indiscriminate use of firepower.
"This will result in increased civilian casualties, disruption of Ukrainian infrastructure and intensify the humanitarian crisis."
Meanwhile, Foreign Secretary Liz Truss has said that she fears peace talks between Russia and Ukraine are being used as a "smokescreen" by the Kremlin to allow forces to regroup.
In an interview with The Times, Truss said: "If a country is serious about negotiations, it doesn't indiscriminately bomb civilians that day."
The minister said she was "very sceptical" about the negotiations, and added: "What we've seen is an attempt to create space for the Russians to regroup."
She said: "We don't see any serious withdrawal of Russian troops or any serious proposals on the table."
Truss said that "the Russians have lied and lied and lied. I fear the negotiation is yet another attempt to create a diversion and create a smokescreen".
At the same time, Putin is tightening his control over Russian media in a bid to disguise the heavy losses his forces have suffered in battle, Gen Hockenhull said.
"The Kremlin is attempting to control the narrative, hide operational problems and obscure high Russian casualty numbers from the Russian people," he said.
As many as 7,000 Russian troops have died so far in the fighting, including four major generals and a number of other senior officers, American sources estimate.
Russian forces have been continuing to pound the outskirts of the capital Kyiv, while the cities of Kharkiv, Sumy and Mariupol, where more than 2,500 civilians are reported to have died, remain under siege.
Earlier another missile strike was reported on an aircraft maintenance plant on the outskirts of the western Ukrainian city of Lviv, which has largely so far escaped the fighting.
Western officials are concerned that as Russia uses up its stock of precision weapons, it will turn increasingly to "dumb" bombs resulting in even more casualties among the civilian population.
"There is an enormous amount of artillery ammunition which the Russian forces hold. It may be a logistical challenge for them to get all of it in place," one official said.
"But if they are able to get those supplies forward then they could mount that artillery bombardment for a very, very considerable period of time. Those bombardments could be very intense."
UK armed forces minister James Heappey said the indiscriminate shelling of cities was "very probably" a war crime and that the Russian president bore ultimate "culpability" for atrocities.
He warned, however, that everyone in the military chain of command could be held responsible for war crimes they were involved in.
"The evidence being gathered points very much towards war crimes being committed in Ukraine," he told Sky News.
"The culpability for war crimes sits absolutely with the leader of the Russian government, the man who decided to do all of this in the first place," he said.
"It is not just Putin who ends up being responsible for war crimes as and when the evidence is gathered and people are held to account.
"They too are involved in the prosecution of war crimes in Ukraine. This is a stain on the Russian nation."
