WASHINGTON (WASHINGTON POST) - Hours before departing on Wednesday (June 26) for a global economic summit in Japan, United States President Donald Trump complained about the military alliance between the two countries, criticised world leaders he will see at the gathering and unloaded on a list of domestic political foes, including a star of the US women's soccer team.

Mr Trump's comments during a wide-ranging Fox Business interview and on Twitter came as he prepared to discuss a range of pressing issues at the two-day Group of 20 meeting in Osaka, including trade disagreements, escalating tensions in the Middle East and stalled nuclear negotiations with North Korea.

"If Japan is attacked, we will fight World War III," he said on Fox Business. "We will go in, and we will protect them, and we will fight with our lives and with our treasure. We will fight at all costs. But if we're attacked, Japan doesn't have to help us at all. They should watch it on a Sony television, the attack."

In the same interview, he berated China over stalled trade talks and falsely said China is bearing the full brunt of US tariffs imposed this year, despite the effect they are having on US consumers.

"Don't play. Don't let anyone tell you that China's not paying for it. China's paying for it," Mr Trump said. "We're not paying for any of it."

He also criticised or insulted European leaders he will see at the Japan meeting, renewed a tariff threat against Europe, denounced his handpicked Federal Reserve chairman and complained that Vietnam is "almost the single worst abuser of everybody".

"Almost all countries in this world take tremendous advantage of the United States," he said. "It's unbelievable."

The comments fit with Mr Trump's habit of criticising or mischaracterising his counterparts before global summits, remarks that often put US allies on edge and scramble the potential for progress.

Mr Trump dislikes set-piece group sessions such as the G-20, whose chief value is often symbolic, and he frequently uses them to make vague and often inaccurate complaints that other nations take advantage of the US in trade and defence.

Just hours before the Group of Seven summit in Canada last June, he attacked Canadian leader Justin Trudeau and French leader Emmanuel Macron and suggested that Russia should be invited to the exclusive forum, which was designed in part to specifically exclude leaders from Moscow.

And hours after that summit, Mr Trump exploded in anger at Mr Trudeau and announced that he was withdrawing from a joint statement with the other countries.

Mr Trump's comments on Wednesday on Japanese defence refer to a treaty signed more than 60 years ago that forms the foundation of a post-World War II alliance.

Bloomberg reported this week that he had recently mused to confidants about withdrawing from the treaty, citing people familiar with the matter. Administration officials consider such a move highly unlikely.

Mt Trump is set to meet Japanese Prime Minister Shinzo Abe while in Osaka, and his comments about the military agreement could add a bit of awkwardness.

He is also expected to meet Chinese President Xi Jinping on the sidelines of the gathering and hold a similar separate meeting with Russian President Vladimir Putin.

Mr Trump said on Fox News that if he does not cut a deal with Mr Xi, he plans to proceed with tariffs on even more Chinese imports to the US. Mr Trump has tried to use the threat of tariffs as a way to force foreign leaders to make concessions on trade deals.

But for the first time on Wednesday, Mr Trump said he would seek a 10 per cent import penalty and not the 25 per cent tariff he had proposed. He has already imposed a 25 per cent tariff on US$250 billion (S$338.7 billion) in Chinese imports. He has threatened to penalise an extra US$300 billion, though in the Fox News interview he said he would penalise an extra US$600 billion in imports. It is unclear where he got that figure, though he could have been adding together the entire universe of goods annually imported to the US from China.

Mr Trump will visit South Korea on the trip and is widely expected to use the stop to visit the demilitarised zone between South Korea and North Korea.

He had planned a secret visit to the heavily fortified border zone in 2017, in the midst of his escalating war of words with North Korean leader Kim Jong Un, but it was cancelled because of poor weather.

As he left the White House on Wednesday, Mr Trump told reporters that he would meet "a lot of other people" while in Asia. While no third summit with Mr Kim is contemplated on this trip, Mr Trump said the two, who now claim a warm mutual understanding, might meet in a "different form".

Elsewhere during the hours leading up to his departure for the economic summit, Mr Trump without evidence accused former special counsel Robert Mueller of committing a crime, revisited complaints against his 2016 political opponent Hillary Clinton and repeated unproven claims about FBI "spying" on his campaign.

Mr Trump also took what appeared to be another swipe at the late senator John McCain, telling a crowd in Washington that senators who had given him a "hard time" have "gone on to greener pastures - or perhaps far less green pastures".

"We had a little hard time with a couple of them, right?" Mr Trump said of Republican senators who voted against him on repealing the Affordable Care Act in 2017. "Fortunately, they're gone now," he said in campaign-style remarks to a largely evangelical Christian audience at the Faith and Freedom Coalition.

Mr McCain died of brain cancer last year. The other two Republicans who defied Mr Trump's wishes on the healthcare vote, Senators Susan Collins of Maine and Lisa Murkowski of Alaska, remain in office.

White House spokesman Hogan Gidley in a statement denied that Mr Trump was referring to Mr McCain and said he was targeting former Republican senators Jeff Flake and Bob Corker. Both voted to repeal Obamacare.

Mr Trump also criticised US soccer star Megan Rapinoe, who said she would not accept an invitation to visit the White House while he is president, and he implied that NBA team owners and players should treat him better because he has lowered black unemployment and supports criminal-justice legislation.

Rapinoe "should WIN first before she TALKS! Finish the job!" Mr Trump wrote on Twitter.

"We haven't yet invited Megan or the team, but I am now inviting the TEAM, win or lose," he wrote. "Megan should never disrespect our Country, the White House, or our Flag, especially since so much has been done for her & the team. Be proud of the Flag that you wear. The USA is doing GREAT!"

He also used the Fox interview to complain that the Fed is not lowering interest rates, which remain near historic lows, saying it is putting the US at a disadvantage against other countries. He suggested Fed chairman Jerome Powell, whom he nominated for the job but has heavily criticised since, is motivated by his eagerness to show that he will not be bullied by him.

"Now he's trying to prove how tough he is, because he's not going to get pushed around," the president said. "Here's a guy - nobody ever heard of him before, and now I made him and he wants to show how tough he is, OK? Let him show how tough he is. He's a - he's not doing a good job, OK, let me - let me be nice about it."