President Trump and first lady Melania Trump welcome Irish Prime Minister Leo Varadkar at the South Portico as he arrives at the White House on Thursday.
"If they don't talk to us we're going to do something that's very severe economically", Trump said.
"Today's comments should not be a surprise, as President Trump has been very clear for years on Brexit".
Trump did state his belief that Varadkar is now in a "complicated position" due to Brexit and stated his belief that the European Union has treated the U.S. "very unfairly" over the years.More news: Developer Of 2D Beat 'Em Up Castle Crashers Teases Switch Release
Theresa May has previously revealed that Mr Trump told her to "sue the EU" and "not go into negotiations".
Mr Trump was asked if he was still a Brexit supporter.
The president said he and Varadkar discussed the issue during their Oval Office meeting.
Turning to Mr Varadkar, he said: "Leo, I'm sure you agree on that". "I regret that Brexit's happening and the United Kingdom was a really important part of the European Union".More news: Huawei Has an Android Alternative, But It Doesn't Want to Use It
But he was a gracious host at the annual St Patrick's week breakfast and he thanked Mr Varadkar, one of the world's few openly gay leaders, for visiting and "honouring us with your presence".
He lamented that the country is "being ripped apart" because of a long-standing political crisis that gripped the government and parliament. Varadkar said he appreciates what the president has done for his country economically. I missed it last time but I would have loved to have been there.
"I look forward to talking to you later about Brexit, giving you our perspective on it and the real importance of protecting the Good Friday Agreement and the really hard-won peace in Northern Ireland", Varadkar told Trump, referring to the 1998 British-Irish peace accord.
However, some differences between the two nations' administrations were highlighted on Thursday morning (March 14), when Varadkar arrived at the Naval Observatory for a breakfast meeting with Vice-President Mike Pence - and took along his partner, Matt Barrett.More news: Cyclone Idai could put 'tens of thousands' Mozambicans in danger
During the annual event, the Taoiseach also presented the president with a bowl of shamrock to mark his St Patrick's visit to Washington DC. He was proud to be an American, but I can still hear that Irish brogue in my heart when my grandfather used to talk about the old country and his home across the pond.