When reporters asked Donald on Thursday who he thought was responsible for the poisoning of ex-Russian spy Sergei Skripal and his daughter, Trump replied: "It certainly looks like the Russians were behind it."
Trump labelled the incident, that took place in the British cathedral town of Salisbury, "a very sad situation" during a meeting at the White House with Ireland's prime minister, Leo Varadkar.
The president said he had been in contact with British Prime Minister Theresa May about the incident and said the US was taking it "very seriously".
It came the same day as Russia's Foreign Minister Sergei Lavrov said the motive behind the attack may have been to complicate Russia's hosting of this summer's soccer World Cup, denying his country had anything to do with it.
Lavrov added that he hoped Skripal recovered from the nerve agent attack so that he could shed light on what happened, Russian state news agency TASS reported.
The UK, US, France and Germany issued a joint statement over the Salisbury attack on Wednesday calling it “an assault on UK sovereignty”.
They condemned Russia and said it was a breach of international law that “threatens the security of us all”.
Russia's embassy to the UK on the same day posted a picture of a thermometer in icy conditions with the caption: "The temperature of UK-Russia relations drops to -23 but we are not afraid of cold weather.
The number 23 apparently referencing the number of Russian diplomats British Prime Minister Theresa May decided to eject from the UK, who she said were spies working under diplomatic cover.
It was the biggest expulsion of Russian diplomats from London since the Cold War.
Lavrov said Russia's response would come "very soon".