ABOARD AIR FORCE ONE--U.S. President Donald Trump said on Friday the United States and Japan have begun discussion over trade, saying that Tokyo "knows it's a big problem" if an agreement cannot be reached.

"We're starting that," Trump told reporters aboard Air Force One. "In fact Japan has called us ... they came last week."

"If we don't make a deal with Japan, Japan knows it's a big problem," he added.

Trump, who is already challenging China, Mexico, Canada and the European Union on trade issues, has expressed displeasure about his country's large trade deficit with Japan, but had not asked Tokyo to take specific steps to address the imbalance.

On Thursday, though, CNBC reported he had told a Wall Street Journal columnist he might take on trade issues with Japan, causing the dollar to slip against the yen.

The White House said Trump would push for fair trade.

"The president has been clear that he will fight to promote free, fair, and reciprocal trade with countries around the world, including Japan, that impose a range of restrictions on U.S. market access," White House spokeswoman Lindsay Walters said in a statement.

"The United States and Japan have been in close contact on ways to address such barriers, including through the U.S.-Japan Economic Dialogue."