Markets have been buoyed this morning by a vague promise from US President Donald Trump to bring in "phenomenal" tax reforms in the coming weeks.

"That's coming along very well. We're way ahead of schedule," he said after a meeting with airline bosses.

"We're going to be announcing something I would say over the next two or three weeks that will be phenomenal in terms of tax and developing our aviation infrastructure," the republican added.

White House spokesperson Sean Spicer offered further details, saying that the policies will be, "A comprehensive plan, something we haven’t seen since 1986 ... It’s going to recognise the need to give so many working Americans the relief that they need."

"What he wants to do is create a tax climate that not only keeps jobs here but incentivises companies to want to come here, to grow here, to create jobs here, to bring their profits back here," he continued.

One of Trump's election promises was a plan to slash standard business taxes from 35% to 15%.

He blamed the exodus of US firms to counties like Ireland on the high tax rates levied in the US.

Significant changes to tax policy is likely to face opposition in Congress - it needs to approve tax changes.