The Taoiseach has said there is "no doubt" the decision by US President Donald Trump to pause military aid to Ukraine is "a very, very serious development".
A White House official said yesterday that the "President has been clear that he is focused on peace. We need our partners to be committed to that goal as well.
"We are pausing and reviewing our aid [to Ukraine] to ensure that it is contributing to a solution," the official added.
Micheál Martin said it is "a serious setback" but said there "needs to be continued engagement between everybody".
"It's a very serious development. There's no point in saying anything else."
Speaking as he arrived at Government Buildings, Mr Martin said while everyone supports the broad push for peace in Ukraine, it has to be "a sustainable and durable peace".
He said he will discuss the matter at a meeting tomorrow night with British Prime Minister Keir Starmer, who he said has shown "very good leadership" on the issue.
The Taoiseach, who will also be meeting with European leaders this week, said he believes they "will see now how we can sort of move forward this entire situation in terms of engagement with the US, and also then in terms of what the EU itself can do".
He added: "Despite the fact, we really haven't had an indication from Russia as to whether it is up for a durable, sustainable peace; in any event, I think Europe will not stop in endeavouring to move this forward in a more positive light than currently is the situation."
Mr Martin said he will be "pushing that we do everything we possibly can to continue our engagement with the United States government with a view to identifying a pathway forward here that protects Ukrainian sovereignty and its territorial integrity".
The Tánaiste and Minister for Foreign Affairs has said that at a time when the EU and the US "need to be moving closer together in a way forward, it is extraordinarily concerning that it seems overnight, things are moving further apart".
Simon Harris described the US President's decision as "deeply concerning" and a "worrying development".
He said more details are needed from Mr Trump in relation to his intentions.
Speaking as he arrived at Government Buildings, Mr Harris said: "Up until now, it has always been the situation that aid was continuing to be provided, that equipment and support was continuing to be provided, whilst trying to find a process towards peace."
"To unilaterally and abruptly disrupt that would have very significant consequences," The Tánaiste added.
The Taoiseach said there is "no doubt" that Mr Trump's decision is "a very, very serious development".
Meanwhile, new EU plans to strengthen Europe's defence industry and increase military capabilities could mobilise close to €800 billion, EU Commission President Ursula von der Leyen has said.
The EU will propose to give member states more fiscal space for defence investments, as well as €150bn in loans for those investments, and will aim to mobilise private capital as well, Ms von der Leyen said.
