Ukrainian President Volodomyr Zelenskiy told the UN Security Council on Tuesday that "accountability must be inevitable" for Russia as he accused invading Russian troops of committing "the most terrible war crimes" since World War Two.
The European Union's executive meanwhile proposed sweeping new sanctions against Russia, including a ban on coal imports, as part of the West's response to evidence of civilian killings in the northern Ukrainian town of Bucha retaken from Russian forces.
Between 150 and 300 bodies may be in a mass grave by a church in Bucha, Ukrainian human rights ombudswoman Lyudmyla Denisova said.
Zelenskiy questioned the value of the 15-member Security Council, which has been unable to take any action over Russia's Feb. 24 invasion of Ukraine because permanent member Moscow is a veto power, along with the United States, France, Britain and China.
"We are dealing with a state that turns its veto at the UN Security Council into the right to (cause) death," Zelenskiy said in a live video address from Ukraine's capital Kyiv, urging reform of the world body. "Russia wants to turn Ukraine into silent slaves," he said.
Moscow denies targeting civilians in Ukraine and has said the deaths in Bucha were a "monstrous forgery" staged by the West to discredit it.
Responding to Zelenskiy's address, Russia's UN Ambassador Vassily Nebenzia told the Council: "We've heard once again a huge amount of lies about Russian soldiers and military."
Russia says it launched a "special military operation" in Ukraine on Feb. 24 to demilitarize and "denazify" a country that President Vladimir Putin regards as an illegitimate state. The Kremlin's position is rejected by Ukraine, a parliamentary democracy, and the West as a pretext for an unprovoked invasion.
EU eyes widening sanctions
The proposed EU sanctions, which the bloc's 27 member states must approve, would bar Russian imports worth 9 billion euros ($9.84 billion) and exports to Russia worth 10 billion euros, including semiconductors and computers, and stop Russian ships entering EU ports.
European Commission President Ursula von der Leyen said it was working on banning oil imports too.
"We all saw the gruesome pictures from Bucha and other areas from which Russian troops have recently left. These atrocities cannot and will not be left unanswered," she said on Twitter.
Earlier in the day Kyiv Mayor Vitali Klitschko called for all business ties with Russia to be cut so as to halt the flow of "bloody money".
Since Russia shifted its offensive from northern Ukraine, where it failed to take any major cities, to the south and east, grim images have emerged from Bucha near Kyiv, including a mass grave and bound bodies of people shot at close range.
The apparent atrocities have prompted calls for tougher action against Moscow and an international investigation.
Several EU countries, including Germany, France, Italy, Latvia and Estonia, have announced expulsions of Russian diplomats and Moscow said it would respond in kind.
Sanctions more severe than any ever imposed on a major power have isolated and crippled Russia's economy, but Ukraine says the West needs to do much more to starve Moscow's war machine.
"Every euro, every cent that you receive from Russia or that you send to Russia has blood, it is bloody money and the blood of this money is Ukrainian blood, the blood of Ukrainian people," Klitschko told a conference in Geneva via video link.
Europe, which obtains about a third of its natural gas from Russia, has been wary of the economic impact a total ban on Russian energy - which Ukraine maintains is vital to securing a peace deal - would bring.
An EU ban on Russian coal would be worth around 4 billion euros a year, von der Leyen said - tiny in comparison with last year's 100 billion euros in oil and gas imports from Russia.
But signalling strengthening EU resolve, German Foreign Minister Annalena Baerbock said the coal ban was the first step towards an embargo on all fossil fuel imports from Russia.
Zelenskiy said earlier the Kyiv government, which accuses Russia of genocide, had no choice but to negotiate to end the war, now in its sixth week, which has killed thousands, sent millions fleeing and turned cities into rubble.
"All of us, including myself, will perceive even the possibility of negotiations as a challenge," Zelenskiy said in a television interview.
Russian news agency Interfax cited a deputy Russian foreign minister as saying talks, which last convened on Friday, were continuing via video link.
Russia denied that its forces had carried out any atrocities and said earlier it would present "empirical evidence" to the Security Council meeting proving its forces were not involved.
At the weekend, Reuters reporters in Bucha saw several bodies apparently shot at close range, along with makeshift burials and a mass grave, but could not independently verify the number of dead or who was responsible.
Satellite images taken in March and provided to Reuters by US company Maxar Technologies showed bodies of civilians on a street in Bucha, which was occupied by Russian forces until about March 30, undercutting claims that the scenes were staged.
UN aid chief Martin Griffiths, who is seeking a humanitarian truce in Ukraine, told the Security Council that "we have a long road ahead of us".
Battles in the east
Ukraine said it was bracing for about 60,000 Russian reservists to be called in to reinforce Moscow's offensive in the east, where Russia's main targets have included the port of Mariupol and Kharkiv, the country's second largest city.
Ukraine's general staff said Russian forces aimed to fully take over the Donetsk and Luhansk provinces claimed by Russian-backed separatists and encircle a group of Ukrainian forces.
"Russian troops have attacked Mykolayiv with cluster munitions banned by the Geneva Convention. Whole blocks of civilian buildings have come under fire, in particular, a children's hospital. There are dead and wounded, including children," the general staff said in a daily update on Tuesday.
Reuters could not independently verify the claims.