US President Donald Trump wants a new 20 per cent tax on all imports from Mexico to pay for a wall on the southern US border.

No details were available on how the tax would work but White House spokesman Sean Spicer told reporters today that Trump wanted it to be part of a tax reform package that the US Congress is contemplating.

Senate Republican leader Mitch McConnell says Congress is moving ahead with the wall that would cost US$12 billion to US$15 billion ($16.5 billion to A$20.6 billion).

McConnell and House of Representatives Speaker Paul Ryan did not say how Congress would pay for the wall.


"We anticipate a supplemental (budget) coming from the administration," Ryan said at a news conference.

"The point is we're going to finance the Secure Fence Act."