Two Republicans have signed a letter urging US President Donald Trump to be persuaded to hand over his tax returns.

Mark Sanford and Walter Jones put their names to a letter that requested both the Senate Finance Committee and the House Ways and Means Committee to request the president's tax documents.

The letter, seen by The Hill, reads: "Disclosure would serve the public interest of clarifying President Trump's conflicts of interest in office, the potential for him to personally benefit from tax reform, and ensure that he is not receiving any preferential treatment from the IRS."

The majority of the House Democratic Caucus signed the letter, which was put together by Democrat Bill Pascrell but the signatures of two GOP members is sure to cause a stir.

Trump's failure to turn over his tax returns during his presidential campaign was unusual and controversial. He claimed at the time that he could not release the accounts because he was being audited.

The president also made the unusual response to allegations that he had not paid federal taxes for 18 years made him "smart".

It is now clear that some within the GOP, as well as the Democratic Party, wish the president to be more open about his tax affairs although the committees may not comply with their request.

Last month, the chairman of the House Ways and Means Committee Kevin Brady said: "My belief is that if Congress begins to use its powers to rummage around in the tax returns of a president, what prevents Congress from doing the same to average Americans?"