Trump tax reform to be 'largest ever'

US President Donald Trump will propose the "largest tax reform" in US history, Treasury Secretary Mnuchin says.

Mnuchin told CNBC that there was fundamental agreement with Congress on the goals of tax reforms and that the administration would release details of the plan later on Wednesday (Thursday AEDT).

"This is going to be the biggest tax cut and the largest tax reform in the history of our country," Mnuchin told a news forum in Washington.

The top tax rate for individuals would fall by a few percentage points, from 39.6 percent to the "mid-30s." That's according to an official familiar with the plan.

White House officials already have said the top corporate tax rate would drop from 35 per cent to 15 per cent under Trump's plan.

Separately, House of Representatives Speaker Paul Ryan said he had seen a "sneak preview" of the plan.

"We like it a lot, it puts us on the same page, we're in agreement on 80 per cent and on the 20 per cent we're in the same ballpark," Ryan said.