Japan has conveyed to the United States its objection to President Donald Trump's idea of adding South Korea to the Group of Seven summit, Japan's Kyodo News reported Sunday citing diplomatic sources.

In late May, Trump said he would postpone the G7 summit until at least September and expand the list of invitees to include Australia, Russia, South Korea and India.

Kyodo reported that Japan conveyed the objection immediately after Trump raised the prospect, saying that South Korea is not "in lockstep with G7 members on China and North Korea (Democratic People's Republic of Korea) issues."

According to the sources, the Japanese government has refrained from publicly expressing its opposition to South Korea's participation, thinking that the G7 expansion proposal may be just "a thought" of Trump that could fizzle out.

The G7 groups the United States, Britain, France, Japan, Germany, Italy and Canada, and the European Union also attends.

Japan's top government spokesman Yoshihide Suga said at a press conference on June 1 that the G7 is an "important framework" for coordination in tackling global challenges.

Asked whether the U.S. government had informed Japan of Trump's G7 expansion plan, Suga said "I will refrain from speaking about the details, but Japan and the United States are engaged in close communication."

Trump's suggestion that the G7 could be expanded to include Russia was quickly rejected by Canada's Prime Minister Justin Trudeau and the European Union.