Taoiseach Micheál Martin has told the UN General Assembly that Russia is behaving as a "rogue state".
In his speech tonight, Mr Martin said the discovery of mass civilian graves, targeting of nuclear facilities and civilian infrastructure along with President Putin's plan for "sham referenda in eastern Ukraine" show Russia behaving as a rogue state.
"In Ukraine in July, I heard first-hand accounts from civilians of the brutality and violence visited upon men, women and children by occupying Russian forces.
"And where Russia's forces have been pushed back, we have seen the wanton destruction, and the uncovering of mass civilian graves, such as in Bucha and more recently in Izium.
"We have seen the targeting of nuclear facilities and of civilian infrastructure.
"And now we see President Putin plan sham referenda in eastern Ukraine, aimed at forcibly changing Ukraine’s borders, in clear violation of the UN Charter.
"We have to name what we are seeing. These actions, taken collectively, show Russia behaving as a rogue state," Mr Martin said.
The Taoiseach said: "We face an expansionist power, brutally invading and occupying a peaceful neighbour.
"We faced this many times in Europe in the 20th century. We did not think we would face it again in the 21st century."
Mr Martin said that this "is not just a European issue" and "not just a concern for 'the West'".
He said that all states, particularly small countries like Ireland "should fear a world where might equals right, where the strong can bully the weak; where sovereignty and territorial integrity can be blatantly violated; and where the UN Charter – the Charter that all of us in this Assembly have faithfully put our trust in – can be flouted with impunity".
Earlier, World leaders at the United Nations called for Moscow to be held accountable for human rights violations in Ukraine as Russian Foreign Minister Sergei Lavrov defended Moscow's war and accused its neighbour of committing atrocities.
Addressing a meeting of the UN Security Council on atrocities committed in Ukraine since Russia's 24 February invasion, Mr Lavrov accused Ukraine of creating threats against Russian security and "brazenly trampling" the rights of Russians and Russian-speakers in Ukraine.
"I can assure you that we will never accept this," said Mr Lavrov, who came to the council chamber to speak and then left.
"Everything I've said today simply confirms that the decision to conduct the special military operation was inevitable."
He said countries supplying weapons to Ukraine and training its soldiers were parties to the conflict, adding that "the intentional fomenting of this conflict by the collective West remained unpunished."
Mr Lavrov also clashed with Minister for Foreign Affairs Simon Coveney over the issue of war crimes.
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Minster Coveney strongly supported efforts to hold Russia accountable for war crimes, and the crime of aggression against another country. He spoke of his own experiences seeing the bodies of victims in Bucha.
But Mr Lavrov said Bucha was a staged incident, and that nobody except Russia has spoken about it since March.
He defended Russia's actions as a protection for Russian language rights, and said the Ukrainian government was a neo-nazi conspriracy that came to power in a coup d'etat backed by the West.
US Secretary of State Antony Blinken pledged that Washington would continue to support Ukraine to defend itself.
"The very international order we've gathered here to up hold is being shredded before our eyes. We can't let President Putin get away with it," Mr Blinken told the council.
Thousands have been killed and Ukrainian cities reduced to rubble since Russian forces invaded Ukraine in the biggest assault on a European state since World War Two.
The Security Council meeting today took place during the annual gathering of world leaders for the UN General Assembly.
The council has been unable to take any meaningful action on Ukraine because Russia is a permanent veto-wielding member along with the US, France, Britain and China. Today's meeting is at least the 20th time the Security Council has met on Ukraine this year.
International Criminal Court (ICC) Prosecutor Karim Khan told the council there were "reasonable grounds" to believe crimes within the jurisdiction of the court had been committed in Ukraine. The Hague-based court handles war crimes, crimes against humanity, genocide and crimes of aggression.
Mr Khan said the ICC investigation priorities were intentional targeting of civilian objects and the transfer of populations from Ukraine, including children.
The US has said estimates from a variety of sources, including Moscow, indicate that authorities have "interrogated, detained, and forcibly deported" up to 1.6million Ukrainians to Russia since Moscow's invasion.
The US, UK and other members called for nations to ensure Russia is held accountable for atrocities they say Moscow has carried out in Ukraine.
"We must make clear to President Putin that his attack on the Ukrainian people must stop ... that there can be no impunity for those perpetrating atrocities," British Foreign Secretary James Cleverly said, adding that the world needed to reject Moscow's "catalogue of lies".
UN chief Antonio Guterres told the meeting that talk of a nuclear conflict is "totally unacceptable".
Yesterday, Russian President Vladimir Putin threatened to use nuclear weapons to defend Russia.
Mr Guterres told the council ministerial meeting that he was concerned about the plans for "so-called 'referenda'."
"Any annexation of a state's territory by another state resulting from the threat or use of force is a violation of the UN Charter and of international law," Mr Guterres said.
Referendums on joining Russia are due to take place from Friday until Tuesday in several largely Russian-held regions in eastern and southern Ukraine, which comprise around 15% of the country's territory.
China's Foreign Minister Wang Yi said the priority was to resume dialogue without pre-conditions and for both sides to exercise restraint and not escalate tensions.
"China's position on Ukraine is clear. The sovereignty, territorial integrity of all countries should be respected and the reasonable security concerns of all countries should betaken seriously," Mr Wang said.
Asked if he might speak with Mr Lavrov at the Security Council today, Ukraine's Foreign Minister Dmytro Kuleba said he will "keep safe social distance.
Yesterday, Ukrainian President Volodymyr Zelensky, who addressed the General Assembly, demanded a special tribunal and "punishment" for Russia over its invasion and abuses.
Mr Zelensky said that Ukraine wanted "just punishment" for crimes Russia has committed against his nation.
Additional reporting Sean Whelan, AFP and Reuters