Barack Obama has called for international tax reform in the wake of the revelations contained in the Panama Papers.

"There is no doubt that the problem of global tax avoidance generally is a huge problem," he told reporters at the White House on Tuesday. "The problem is that a lot of this stuff is legal, not illegal."

The US president said the leak from Panama illustrated the scale of tax avoidance involving Fortune 500 companies and running into trillions of dollars worldwide.

"We shouldn't make it legal to engage in transactions just to avoid taxes," he added, praising instead "the basic principle of making sure everyone pays their fair share".

Obama described the Panama revelations as "important stuff" and highlighting the impact upon ordinary citizens, added that "a lot of these loopholes come at the expense of middle-class families, because that lost revenue has to be made up somewhere.