Chinese President Xi Jinping has arrived in Osaka ahead of what promises to be a stormy G20 meeting over trade, as US President Donald Trump hit out at Beijing and vowed more tariffs.

The long-running trade war between the world's top two economies looked poised to dominate the agenda of the group of 20 summit in Osaka from tomorrow and Mr Trump struck a characteristically bullish tone on departure.

"China's economy is going down the tubes - they want to make a deal," Mr Trump said in an interview with Fox Business Network.

Furious at what he sees as an unfair advantage in the trading system, Mr Trump has already hit Beijing with $200 billion in levies on Chinese imports and appeared to threaten more.

"You have another $325 billion that I haven't taxed yet - it's ripe for taxing, for putting tariffs on," he said in his interview.

Global markets are hoping the US and China can seal a truce to the trade conflict that has proved a major headwind to the world economy.

Most experts say a formal deal is unlikely at the G20 summit due to a lack of time to prepare the complex issues involved and believe a ceasefire and commitment to keep talking is the most likely outcome.

While US-China trade is likely to dominate the meeting, world leaders are also facing a perfect storm of geopolitical hotspots from Iran to North Korea and Venezuela.

Tensions with Tehran are at a high after Mr Trump pulled back from military action at the last minute in response to the downing of an unmanned US drone.

Before leaving for the G20, Mr Trump said he did not seek conflict with the long-time US foe but warned it "wouldn't last very long" if war did break out.

President Hassan Rouhani had earlier tried to rein in the crisis, saying that Iran "never seeks war" with the United States.

Iran will be one of several topics when Mr Trump meets his Russian counterpart Vladimir Putin on the sidelines of the meeting, along with arms control, the crisis in Venezuela, and Ukraine.

The keenly awaited meeting is overshadowed by a probe into Mr Trump's relations with Russia and controversy that erupted the last time he held talks with the Kremlin leader in Finland in 2018.

"I'll have a very good conversation with him," Mr Trump told reporters at the White House. "What I say to him is none of your business."

Hosts Japan have desperately tried to steer attention away from the Trump show and on to the formal aspects of their G20 agenda, including the challenges posed by ageing populations and climate change.

"Confrontation tends to attract attention... but Japan, as the chair, hopes to find common ground rather than differences in opinions," said Prime Minister Shinzo Abe as he left Tokyo for Osaka.

But tensions over the final statement on climate change are likely to burst into the open, with a German source saying it was "particularly difficult this year" to reach agreement.

And Mr Trump appeared in no mood for compromise, hitting out at traditional allies one-by-one, even his hosts.

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

He tweeted that tariffs imposed by India were "unacceptable", described Vietnam as the "single worst abuser" on trade, lashed Germany as "delinquent" and mocked Japan, which has been under a US military umbrella since World War II.

"If Japan is attacked, we will fight World War III. We will go in and protect them with our lives and with our treasure," he told Fox Business Network.

"But if we're attacked, Japan doesn't have to help us. They can watch it on a Sony television."