US Treasury Secretary Steven Mnuchin said Friday that he will not comply with a congressional subpoena to turn over President Donald Trump's federal tax returns in six years, likely to trigger a court battle with Congress.

In a letter sent to House Ways and Means Committee Chairman Richard Neal, Mnuchin said that the request "lacks a legitimate legislative purpose" and that he would not authorize the release of Trump's personal and business tax returns to Congress.

Tax transparency has been in the spotlight after Trump rejected a decades-old tradition that presidents and White House hopefuls would make their tax returns public.

Democrats have claimed that Trump's business dealings present a host of potential conflicts of interest that they can't understand without his tax information.