United States President Donald Trump agreed to honour the longstanding "One China" policy during a phone call with Chinese President Xi Jinping, the White House says, taking steps to improve ties after after angering Beijing by talking to the leader of Taiwan.

Mr Trump further unnerved Beijing over the self-ruled island in December, saying the United States did not necessarily have to stick to the policy, under which Washington acknowledges China's position that Taiwan is part of "One China".

Diplomatic sources in Beijing said China had been nervous about Mr Xi being left humiliated in the event a call with Mr Trump went wrong and the details were leaked to the media.

Last week, US ties with staunch ally Australia became strained after the Washington Post published details about an acrimonious phone call between Mr Trump and Prime Minister Malcolm Turnbull.

The two sides also signalled that with the "One China" issue resolved, they could have more normal relations.

The two leaders had not spoken by telephone since Mr Trump took office on January 20.

A White House statement said Mr Trump and Mr Xi had a lengthy phone conversation on Thursday night (Washington time).

It came hours before Mr Trump was to play host to Japanese Prime Minister Shinzo Abe at the White House and later, at his Palm Beach retreat.

"President Trump agreed, at the request of President Xi, to honour our 'One China' policy," the statement said.

The statement marked a return to a more traditional US policy after Mr Trump had told The Wall Street Journal last month that the "One China" policy was up for negotiation.

"Representatives of the United States and China will engage in discussions and negotiations on various issues of mutual interest," the statement said.

"This is an important step," said Bonnie Glaser, senior adviser on Asia at the Centre for Strategic and International Studies in Washington, DC.

"This will now pave the way for the engagement of the US and Chinese Governments on a wide range of issues."

The policy, which has been in place since 1979, requires Washington to maintain only unofficial ties with Taiwan, the self-governing island that China claims as its own territory.

China views any hint of official US recognition of the island as anathema to China's revival as a great Asian power.

Some had questioned why Mr Trump took so long to call Xi, given that he had already spoken with more than a dozen world leaders.

Chinese observers also noted that Mr Trump broke with his predecessors in not extending good wishes on the Lunar New Year holiday last month.

Tensions aplenty

The move comes at a time of uncertainty in US-China relations.

Trump has accused Beijing of cheating in trade, criticised China's military build-up in the South China Sea and said the country is doing too little to pressure North Korea over its nuclear and missile programs.

He has accused China of manipulating its currency to keep its exports inexpensive and threatened a 45 per cent tax on Chinese imports, potentially setting off a trade war.

Underscoring the uncertainties, the call between the leaders came as the US Pacific Command reported a Chinese KJ-200 early warning aircraft and a US Navy patrol plane had an "unsafe" encounter over the South China Sea this week, without elaborating.

The Chinese defence ministry has not commented, but Beijing routinely complains about US military surveillance missions near China.

In a statement read out on Chinese state television, Mr Xi said China appreciated Mr Trump's upholding of the "One China" policy.

"I believe that the United States and China are cooperative partners, and through joint efforts we can push bilateral relations to a historic new high," the statement cited Mr Xi as saying.

The White House described the call as "extremely cordial" with both leaders expressing best wishes to their peoples.

"They also extended invitations to meet in their respective countries. President Trump and President Xi look forward to further talks with very successful outcomes," the statement said.