US President Donald Trump says he told Japanese Prime Minister Shinzo Abe that a decades-old security treaty between the countries must be changed, reiterating his criticism of the pact as unfair.
Trump said he was not planning to withdraw from the treaty, which the partners have long called a linchpin of Asia-Pacific stability.
But he felt it placed too great a burden on the US.
"I told him, we'll have to change it," Trump told a news conference after a two-day G20 summit in Osaka.
"I said, look, if someone attacks Japan, we go after them and we are in a battle, full force, in effect.
"If somebody should attack the United States, they don't have to do that. That's unfair."
The treaty, signed after Japan's surrender in World War Two, commits the US to defend Japan.
In return, Japan provides military bases that Washington uses to project power deep into Asia, including the biggest concentration of US Marines overseas on Okinawa and the forward deployment of an aircraft carrier strike group at the Yokosuka naval base near Tokyo.
An end to the security pact is widely seen as raising the risk of forcing Washington to withdraw a major portion of its military forces from Asia at a time when China's military power is growing.
