US President Donald Trump's administration unveiled the principles of the long-awaited tax reform plan that will significantly cut income taxes for Americans and corporates, media reports said.

"The plan would reduce the number of personal income tax brackets to three from seven, and it would cut the top individual income tax rate to 35 per cent from 39.6 per cent," Xinhua news agency quoted Director of the White House National Economic Council (NEC) Gary Cohn as saying on Wednesday.

At the same time, the plan would cut the corporate income tax rate to 15 per cent from 35 per cent, and it would also impose a one-time tax on overseas profits of US companies, Treasury Secretary Steven Mnuchin said.

That "will bring back trillions of dollars that are offshore to be invested here in the United States, to purchase capital and to create jobs," he said.

The plan would also eliminate all tax deductions for individuals except from mortgage interests and charitable tax deductions, according to Mnuchin.

"We have a once-in-a-generation opportunity to do something really big," Cohn said, noting President Donald Trump has made tax reform a priority and the Republican-controlled Congress also wants to get it done.

However, it's still unclear whether the plan, what US officials called the "biggest tax cut" in history, would be revenue-neutral or would significantly increase fiscal deficits.

Mnuchin has repeatedly argued that the administration would count on the compounding effect of strong economic growth over the next decade to finance the tax cuts.

"This will pay for itself with growth and with reduction of different deductions and closing loopholes," he said.

However, many economists are skeptical about that argument, as the US economic growth will average about only 1.9 per cent over the next ten years, according to the nonpartisan Congressional Budget Office.

The details of a complete tax proposal probably won't be ready until June, According to Mick Mulvaney, director of the White House Office of Management and Budget.