Former foreign minister Alexander Downer has rejected claims he was "directed" to contact an aide to Donald Trump during the US President's bid for power three years ago, countering new assertions in a letter that demands Australian help for a divisive US investigation.

Mr Downer disputed the claim from one of Mr Trump's top Republican allies after Senator Lindsey Graham issued a letter to Prime Minister Scott Morrison and other national leaders asking for their "continued cooperation" with the probe.

President Donald Trump asked Prime Minister Scott Morrison to help with a probe into the 2016 presidential election. Credit:Bloomberg

The Australian ambassador to the United States, Joe Hockey, also rebuffed Senator Graham's account of Mr Downer's part in the US investigation into Russian interference.

Senator Graham, the chair of the Senate Judiciary Committee and a vocal supporter of the President, wrote on Wednesday to Mr Morrison as well as Italian Prime Minister Giuseppe Conte and British Prime Minister Boris Johnson asking them to help the inquiry.