Washington (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.

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