A Japanese newspaper has raised the possibility of Chinese President Xi Jinping also travelling to Singapore when US President Donald Trump meets North Korean leader Kim Jong Un for an unprecedented summit on June 12.

The Mainichi Shimbun's Washington-based correspondent cited American diplomatic sources as saying there was a chance that Mr Xi may meet Mr Trump and Mr Kim in Singapore.

In the report published on Friday (May 11), the Mainichi quoted a senior international negotiator with the National Security Council as telling reporters that "there is a possibility" the leader of a third country may take part.

Mr Trump announced via a tweet on Thursday (May 10) that his summit with Mr Kim will be held on June 12 in Singapore.

As of Friday morning, there were no news reports on North Korean media outlets of the date and venue of Mr Kim's meeting with Mr Trump, Japanese broadcaster NHK reported.

When asked at a regular press briefing on the Chinese president's travel plans, a spokesman for the Chinese Foreign Ministry did not answer the question.

In response to a question posed by Japan's Kyodo news agency whether Mr Xi will be visiting Singapore before, after or on June 12, Mr Geng Shuang replied: "We hope that the meeting of the leaders of the DPRK and the United States will achieve positive results."

When asked by South Korea's Yonhap news agency on China's expectations of the high-stakes summit, Mr Geng said that "China welcomes the positive progress made by the leaders of the DPRK and the United States in preparing for the preparations for the summit".

"We expect that the meeting will achieve positive results, and we will be able to take an important step in pushing forward the denuclearisation of the peninsula, solving the peninsula issue politically, and realising long-term stability of the region," said the Chinese MFA spokesman in Beijing.

At the historic inter-Korea summit on April 27, Mr Kim and South Korean president Moon Jae In issued a joint declaration to say that both sides share the "common goal of realising, through complete denuclearisation, a nuclear-free Korean Peninsula".

They also agreed to pursue three-way talks involving the two Koreas and the US, or four-way talks involving the two Koreas, the US and China, with a view to declaring an end to the war and establishing a permanent and solid peace regime.

Fighting in the Korean War had ended in 1953 with a ceasefire between the United States (representing the United Nations Command), China and North Korea, which has to date not been replaced by a peace treaty.

The Mainichi Shimbun report pointed out that if Mr Xi were to travel to Singapore, it will mean the leaders of the three signatories to the armistice will have gathered in the same country.