The US president, hosting the visiting Japanese Prime Minister Shinzo Abe, also said that if the meeting went ahead but was not working, then he would walk away.
US President Donald Trump, hosting Japan's Prime Minister Shinzo Abe has held a wide-ranging press conference with both men speaking about mutual concerns.
Top of the list was North Korea and a possible upcoming meeting between Trump and Kim Jong-un. But Trump warned it might not happen, or he might walk out.
"Hopefully that meeting will be a great success and we're looking forward to it. It would be a tremendous thing for North Korea and a tremendous thing for the world. So, we will be doing everything possible to make it a worldwide success, not just for the United States or South Korea or North Korea or Japan, but for the entire world.
If I think that it's a meeting that is not going to be fruitful, we're not going to go. If the meeting, when I'm there, is not fruitful, I will respectfully leave the meeting and we'll continue what we're doing, or whatever it is that will continue. But something will happen," he said.
What that something might be remains ambiguous, and keeping people guessing is a pure Donald Trump playbook move.