US President Donald Trump asked Japanese Prime Minister Sanae Takiachi to avoid further escalation of a dispute with China during talks this week, sources with knowledge of the matter said, as he aims to keep a fragile trade war truce with Beijing.

Takaichi touched off the biggest diplomatic dispute with Beijing in years with comments in parliament this month that a hypothetical Chinese attack on Taiwan could trigger Japanese military action, inciting fury in Beijing.

In Tuesday's telephone call with Takaichi, Trump said he did not want to see any further escalation, said the two Japanese government sources, who sought anonymity as the matter is a sensitive one.

Trump made no specific demands of Takaichi, however, one of the sources said, suggesting that he did not echo Beijing's call for a retraction of the comments. Japan has not offered one, saying instead that the remarks reflected longstanding policy.

Japan's Chief Cabinet Secretary Minoru Kihara declined to comment on the reports on Thursday.

Some officials in Tokyo have long worried that Trump may be prepared to weaken support for Taiwan in pursuit of a trade accord with China, a move that may embolden Beijing and spark conflict in East Asia.

"For Trump, what matters most is US-China relations," said Kazuhiro Maejima, a professor of US politics at Sophia University. "Japan has always been treated as a tool or a card to manage that relationship."

Trump's telephone call with Takaichi immediately followed another with Xi Jinping, in which the Chinese president said Taiwan's "return to China" was a key part of Beijing's vision for the world order, the official Xinhua news agency said.

Democratically governed Taiwan, which rejects Beijing's claim of ownership, saying only its people can decide the island's future, has ruled out such a return.

Trump has not commented on the reported discussions with Xi about Taiwan, stressing instead that the world's two largest economies enjoy "extremely strong" relations and are working to finalise a sweeping trade deal.

"The United States' relationship with China is very good, and that’s also very good for Japan, who is our dear and close ally," the White House said in a statement attributed to Trump, following questions from Reuters.

"We signed wonderful trade deals with Japan, China, South Korea, and many other nations, and the world is at peace. Let’s keep it that way!"

Trump's public silence on Japan's escalating dispute with China, in which Beijing has urged citizens not to travel there, dealing a blow to tourism, has further frayed nerves in Tokyo.

The Japanese prime minister's office referred Reuters to its official summary of Takaichi's call with Trump that said the two leaders discussed US-China relations, but did not elaborate.

Trump's request for Takaichi to dial down the volume with Beijing was first reported by the Wall Street Journal.

Trump silent as China pressures Japan

China urged the United States to rein in Japan to prevent "actions to revive militarism", the ruling Communist Party's People's Daily said in an editorial on Thursday that highlighted the role of Japan as their common enemy during World War Two.

"China and the United States share a common responsibility to jointly safeguard the post-war international order and oppose any attempts or actions to revive militarism," it added.

Takaichi's off-the-cuff remark in parliament on Taiwan broke from the strategic ambiguity adopted by her predecessors.

But she cannot retract it, officials previously told Reuters, so that she will struggle to defuse a dispute that could hammer the economy and usher in a long winter in China-Japan ties.

Washington's envoy to Tokyo has said the United States supports Japan in the face of China's "coercion", but two senior lawmakers of its ruling party told Reuters they had hoped for more full-throated support from their top security ally.

Japan hosts the largest overseas contingent of US military, and it has encouraged Tokyo's build-up of defensive capabilities in recent years that has also irked Beijing.

"We’d like a word from Trump himself," said one of the lawmakers, speaking on condition of anonymity. Trump's public silence could be perceived as a green light for Beijing to exert more pressure on Japan, he added.

Some analysts say Trump, however, is more focused on maintaining the fragile truce he struck with Xi last month in the two countries' trade war, as well as avoiding confrontation ahead of a planned visit to Beijing in April.