US President Donald Trump's desire to use Australia and other allies to undermine the probe into Russian links to his 2016 election campaign is damaging America's closest relationships, a top Democrat senator has warned.

Mark Warner, the ranking Democrat on the US Senate Intelligence Committee, said there would be outrage if the situation was reversed.

"Can you imagine if the CIA was asked to provide damaging evidence on a political opponent in Australia?" Senator Warner said in an interview with NBC News on Thursday.

"There would be outrage in our political establishment."

Mr Trump asked Australian Prime Minister Scott Morrison in a recent phone call to help US Attorney General William Barr's investigation into the origins of the FBI probe into foreign influence in the 2016 US presidential election.

"The president contacted me and asked for a point of contact between the Australian government and the US attorney, which I was happy to do on the basis that it was something we had already committed to do," Mr Morrison told Sky News on Wednesday.

Mr Barr has also approached the UK and Italian governments.

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The FBI investigation began in July 2016 two months after then-Australian high commissioner to the UK Alexander Downer met with Mr Trump's foreign relations adviser George Papadopoulos at a London bar.

Mr Downer said Mr Papadopoulos told him Russia had damaging material on Mr Trump's presidential rival Hillary Clinton.

Mr Papadopoulos told AAP he did not disclose the information to Mr Downer.

Mr Downer's tip was passed on to the FBI and the FBI probe led to Special Counsel Robert Mueller's investigation.

Mr Trump has called the FBI and Mueller probes witch hunts.

Senator Warner said the president's outreach to allies threatens to undermine decades of US intelligence sharing, including the Five Eyes relationship between the US, Australia, New Zealand, Canada and the UK.

"Throughout the last 75 years, every administration has valued the sanctity of the Five Eyes relationship," Senator Warner said.

"It appears this administration doesn't value that relationship because if you did, you wouldn't ask your allies for their intelligence for political purposes."

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US intelligence leaders have expressed their concerns to Senator Warner, he said.

"This idea that Australia's intelligence product might be being used to go after a political opponent in domestic American politics ... that's just not what the intelligence community is about," Senator Warner added.