President Donald Trump reaffirmed Washington's "One China" policy in what he said Friday was a "very warm" conversation with Xi Jinping, in an apparent effort to ease tensions after angering Beijing by questioning a major plank of Sino-US relations.

"We had a very, very good talk last night, and discussed a lot of subjects. It was a long talk," Trump told a joint press conference with Japanese Prime Minister Shinzo Abe at his side.

During the phone call, the new Republican president pledged to "honor" a decades-old position that effectively acknowledges Taiwan is not separate from China -- a policy that Trump had suggested a few weeks ago he might jettison.

"President Trump agreed, at the request of President Xi, to honor our 'One China' policy," the White House said in a statement, adding that the two leaders had "extended invitations to meet in their respective countries."

The White House called the phone discussion -- the first since Trump took office -- "extremely cordial," saying the leaders "look forward to further talks with very successful outcomes."

Trump said Friday that he and Xi were "in the process of getting along very well, and I think it will be very much of a benefit to Japan."

Xi, who took the helm of the Communist Party-ruled country in 2012, welcomed Trump's gesture.

"Xi Jinping appreciates Trump's emphasis on the American government's commitment to the One China policy and pointed out that the One China principle is the political foundation of US-China relations," a Chinese foreign ministry statement said.

Trump's insurgent campaign for the White House included frequently lashing out at China, which he accused of currency manipulation and stealing American jobs.

He raised eyebrows in the wake of his election victory with a protocol-busting telephone conversation with Taiwan's President Tsai Ing-wen.

He later threw doubt on the "One China" policy, suggesting that it was up for negotiation and could form part of talks on trade, drawing rebukes from official Chinese media.

- 'Backed Down' -

Ashley Townshend, an expert on US-China relations at the University of Sydney, said Trump's apparent capitulation was an indication of the moderating influence of Secretary of State Rex Tillerson and Defense Secretary James Mattis.

"Everyone will be surprised at the speed with which Trump has backed down on this issue," he said.

The change was unlikely to be conciliatory, he added, but could be read as a sign of pragmatism in the new administration's approach to its powerful adversary.

"There was a real risk prior to this clarification that the two sides would be unable to even find a way to speak," he said.

"This removes an obstacle to relations, but it doesn't advance them in any meaningful way."

Taiwan has been ruled separately since the two sides split in 1949 at the end of a civil war.

Despite having its own government, military and independent foreign policy, Beijing has refused to recognize the island, viewing it as part of its territory awaiting reunification with the mainland -- by force, if necessary.

Washington cut formal ties with Taipei in 1979, when it recognized the Communist mainland rulers in Beijing.

However, the US remains Taiwan's most powerful unofficial ally and its main supplier of arms.

Trump's suggestion that he could restore relations with Taipei -- which Beijing views as a non-starter -- had threatened to chill ties with the Asian giant.

- 'Come to his senses' -

Beijing had been prepared to give Trump-the-candidate a pass, said Wu Xinbo, director of the Center for American Studies at China's Fudan University.

"When Trump tweeted a few things about the 'One China' policy previously, it was prior to his inauguration, so we can consider those his personal opinion," said Wu.

"Now that he is in office, he represents the government's views, and as such, he must emphasize the continuity of policies such as the Taiwan issue and the One China issue."

Xu Guoqi, an expert in Sino-US relations at the University of Hong Kong, said Thursday's call showed the US president had "come to his senses" about a policy that had underpinned ties since Richard Nixon occupied the Oval Office.

"Without honoring the One China policy, the relationship only has one way to go: down to hell," Xu said, adding: "Now the two sides can assume business as usual. They are back to square one."

A high-ranking European diplomat told AFP he hoped that the call indicated an end to Trump's "incoherent signals" on China.

"Maybe the phone call marks the beginning of a more rational and consistent policy," he said.

On Chinese social media, commenters agreed the call was overdue.

"Trump has finally seen the light!" one user posted.

"What could be bad about having good relations with China?"

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