The European Union looks set to impose sanctions on Russia, after President Vladimir Putin declared he recognises two separatist regions in Ukraine as independent.
Mr Putin made the announcement during a live TV address on Monday.
It comes after requests by the leaders of Luhansk and Donetsk - which broke away from Kyiv's control in 2014.
Recognition by the president of their independence could provide a pretext for Russian troops to cross the border into those areas.
And it could further narrow diplomatic options to avoid war, since it would be a rejection of a seven-year-old ceasefire brokered by France and Germany - which is still pushed as the framework for any future negotiations on the crisis.
European Commission President Ursula von der Leyen and Council President Charles Michel condemned the decree "in the strongest possible terms".
"This step is a blatant violation of international law as well as of the Minsk Agreements", the say in a joint statement.
"The union will react with sanctions against those involved in this illegal act.
"The union reiterates its unwavering support to Ukraine's independence, sovereignty and territorial integrity within its internationally recognised borders", they add.
In a statement, Foreign Minister Simon Coveney says this is something Ireland supports.
"Ireland's support for Ukraine's sovereignty and territorial integrity within its internationally recognised borders and its right to choose its own foreign and security policy path is unwavering.
"The decision by the Russian Federation to proceed with the recognition of the non-government controlled areas of the Donetsk and Luhansk oblasts of Ukraine as independent entities contravenes international law, is a blatant violation of Ukraine’s territorial integrity, and marks a clear and unilateral breach of the Minsk Agreements.
"Ireland supports a clear and strong EU response, including additional sanctions measures", he says.
He earlier described it as a "very negative development".
Very negative development. Effectively means Russia is abandoning the Minsk Agreement. https://t.co/xcYFO0pbzv
— Simon Coveney (@simoncoveney) February 21, 2022
There has been fighting in the breakaway regions between pro-Russian separatists and Ukrainian pro-government forces for several years.
More than 14,000 people have been killed since conflict erupted in Donbas - which includes Luhansk and Donetsk - in 2014, shortly after Moscow annexed Ukraine's Crimean Peninsula.
Shelling has intensified since last week along the frontline between the rebels and Ukrainian forces.
On Friday, the rebels started bussing tens of thousands of civilians to Russia, accusing Kiev of planning an attack - which Ukraine denies as propaganda.
Ukraine and the west consider the rebels to be Russia's proxies, and have been warning for weeks that Moscow might use them to construct a case for war.
An estimated 150,000 Russian troops have massed on Ukraine's borders amid fears of an invasion. The US puts the figure at 190,000.
The Russian president denies he is planning to invade his neighbour, saying his forces are there for military drills.
But Moscow has threatened unspecified "military-technical" action unless it receives sweeping security guarantees, including a promise that Ukraine will never join NATO.
At a televised meeting of his security council, which normally meets behind closed doors, Mr Putin restated Russia's demands - insisting it was not enough for the west to say Ukraine was not ready to join NATO at present.
Minister Coveney said on Sunday the prospect of an invasion of Ukraine had become an expectation among world leaders rather than a possibility.
He said the outlook for avoiding war in the region was "pretty bleak" at the moment.
He met with other international ministers and officials at the Munich Security Conference at the weekend, amid escalating tensions in eastern Europe.
While speaking earlier on Monday in Brussels, he said there was still time to de-escalate.
"Clearly there needs to be a message that is fully understood and credible that should there be an invasion of Ukraine that very significant sanctions would follow immediately.
"But I think the way to prevent war is to talk and to find compromise and ways forward that can prevent invasion in the first place".
Additional reporting: IRN