By Dan Merica, CNN

(CNN) -- President Donald Trump, during a meeting Thursday in the Oval Office with Irish Prime Minister Leo Varadkar, told reporters that "it certainly looks like" Russia was behind the nerve agent attack in the United Kingdom.

The attack, which left former Russian spy Sergei Skripal and his daughter Yulia critically ill in the hospital after being exposed, has drawn international condemnation and led British Prime Minister Theresa May to expel 23 Russian diplomats over the poisoning. Kremlin spokesman Dmitry Peskov called the accusations "unfounded" and Russian Foreign Minister Sergey Lavrov said they were "political theater."

"It looks like it," Trump said of whether Russia was behind the attack. "I spoke with the Prime Minister and we are in deep discussions. A very sad situation."

He added: "It certainly looks like the Russians were behind it. Something that should never, ever happen, and we are taking it very seriously, as I think are many others."

A senior national security official mentioned the nerve agent attack in their briefing on new sanctions against Russians entities and individuals.

"The recent use of a military grade nerve agent in an attempt to murder two UK citizens further demonstrate the reckless conduct of its government," the official said.

The comments come in the middle of a day full of events between Trump and Varadkar. After their Oval Office meeting, the men traveled to Capitol Hill for a Friends of Ireland luncheon in the Capitol Building.

The Prime Minister will then return to the White House for the ceremonial Shamrock Bowl Presentation, where Varadkar will present Trump with a crystal bowl full of shamrocks. The tradition -- where the current Prime Minister of Ireland presents the sitting President with a bowl full of shamrocks -- dates back years.

Trump has embraced the tradition that comes with Varadkar's visit. When he greeted the Prime Minister outside the West Wing on Thursday morning, a bushel of clovers was spilling out of his lapel pocket.

During the Oval Office meeting, Trump mentioned he owns a property in Ireland -- Trump International Golf Links & Hotel Doonbeg.

"I have property there and I may never get to see it again," Trump said.

The Prime Minister then mentioned he would be heading to New York to march in the St. Patrick's Day Parade on Saturday.

"That's good," Trump said. "I'd like to do it with you."

"Does it pass Trump Tower?" Varadkar asked.

"It does. It goes right by Trump Tower," Trump said. "I used to watch it all the time. I would watch it all the time."