Trump's staff warned him he was pushing 'completely debunked' conspiracy theories about Ukraine

'I am deeply frustrated with what he and the legal team is doing in repeating that debunked theory to the president'

Donald Trump’s first homeland security adviser says he was “deeply disturbed” by the president’s unfounded claims that Ukraine was responsible for interfering in the 2016 election to support the Democrats.

Thomas Bossert, the former US homeland security adviser to Mr Trump, said he told the president such claims were “completely debunked” before he called Ukrainian President Volodymyr Zelensky and asked him to look into the allegations.

Mr Trump went on to seemingly urge Mr Zelensky to investigate one of his 2020 political rivals, Joe Biden, and look into whether his country was responsible for interfering in the previous US elections.

That phone call — and a resulting whistleblower complaint from a reported member of the US Intelligence community — has provoked an official impeachment inquiry in the House of Representatives.

In an interview with ABC’s This Week, Mr Bossert said about the president’s claims surrounding Ukraine: “It’s not only a conspiracy theory, it is completely debunked.”

“At this point, I am deeply frustrated with what he and the legal team is doing in repeating that debunked theory to the president,” the former official told ABC’s George Stephanopoulos.

“It sticks in his mind when he hears it over and over again,” he added. “And for clarity here, George, let me just again repeat that it has no validity."

It has long been the findings of the US Intelligence Community that Russia was responsible for a multi-pronged attack on the US election system in the 2016 election, and that its influence operations were meant to undermine the candidacy of former Secretary of State Hillary Clinton.

Mr Trump has railed against those facts, denying evidence of interference from the Kremlin as outlined in a sweeping report former Special Counsel Robert Mueller released in April.

The president called Ukraine to look into his unfounded accusations the day after Mr Mueller spoke in public testimony on Capitol Hill about Russian interference in his election victory.

Mr Bossert said he did not believe the president was “pressuring” Mr Zelensky during their phone call, a five-page memorandum of which was released last week.

The memorandum is not a verbatim transcript of the phone call, but details the 25 July conversation between the two presidents.

Mr Trump alleged during the phone call that a hacked server used by the Democratic National Committee in the election was in Ukraine. He urged Mr Zelensky to contact Attorney General William Barr, so they could “get to the bottom of it”.

Democratic leaders have said the phone call was evident of a quid pro quo on the part of Mr Trump, who had reportedly withheld $400m (£324.9m) in aid to the country days before he spoke with Mr Zelensky.