Russia may be preparing a further military incursion into his country, Ukraine's ambassador to the UN in Geneva, Yurii Klymenko, told diplomats at a briefing on human rights.
"There are indications that Russia is on its way to unleash a full blown military intervention in Ukraine's east and south," Klymenko said on Thursday.
His statement was widely supported by other ambassadors, but challenged by a Russian diplomat, who read a prepared statement justifying Russia's actions so far.
Klymenko issued the warning as German Chancellor Angela Merkel said that Europe would step up political and economic sanctions, if Russia goes ahead with its plan to annex Ukraine's Crimean region.
"The EU summit today and tomorrow will make clear that we are ready at any time to introduce phase-3 measures if there is a worsening of the situation," Merkel said, ahead of a European Union meeting on Thursday.
Merkel also said that the Group of Eight would be effectively dead, so long as the diplomatic impasse is not resolved.
Russia, which holds the G8 presidency and is scheduled to host a summit in Sochi in June, has remained defiant, insisting that the legal process to make Crimea part of the federation would be completed this week.
"Practical steps are being taken to implement the agreements on the entry of Crimea and (the Crimean port city of Sevastopol into Russia," Itar-Tass news agency quoted Russian Foreign Minister Sergey Lavrov as saying.
Lavrov also blamed the West in a veiled reference to the US, saying Western nations were trying to "preserve their global leadership and display their exceptionalism rather than striving to be guided by international law."
"The events in Ukraine are a reflection of these approaches," Lavrov said, adding that Moscow would continue to use "political, diplomatic and legal methods" to protect Russians abroad.
Late on Wednesday, the US and Russia engaged in a war of words when Samantha Power, the US ambassador to the UN, likened to the annexation of Crimea to theft, angering her Russian counterpart, Vitaly Churkin, who said the "insults" are endangering their countries' relationship.
"A thief can steal property, but that does not confer the right of ownership on the thief," she had said.
Churkin shot back: "It is simply unacceptable to listen to these insults addressed to our country."
He added, "If the delegation of the United States of America expects our cooperation in the Security Council on other issues, then Mrs. Power must understand this quite clearly."
Churkin did not elaborate. The United States and Russia are the key players in efforts to establish peace talks in Syria, and also are involved in talks over Iran's nuclear program.
Ban heads to Russia
As this developed, UN Secretary-General Ban Ki-moon left for Russia and Ukraine in a bid to seek a diplomatic way out of the crisis, the Associated Press news agency reported.
Ban will meet with President Vladimir Putin and other Russian officials in Moscow on Thursday, said UN spokesman Stephane Dujarric.
On Friday, the UN chief will travel to Kiev for talks with Ukraine's acting president and prime minister.
Meanwhile, the UN council heard a briefing from Ivan Simonovic, assistant UN secretary-general for human rights, who expressed concern over the security of Tatars and other ethnic minorities in Crimea.
He highlighted the disappearance of a Crimean Tatar activist after participating in a March 3 protest. Simonovic said the activist was found dead March 16 and his body bore marks of "mistreatment."
He said the United Nations is deploying a 34-member human rights monitoring mission to Ukraine, scheduled to be in place by Friday.
Simonovic said he was not able visit Crimea because the authorities there refused to receive his mission or ensure its security until it was too late.
But he said he spoke to representatives of displaced Tatars and victims of arbitrary arrests, torture and other human rights violations.