Kim Yong-chol, vice-chairman of the Central Committee of the DPRK's Workers' Party of Korea, headed to Washington on Tuesday, according to the ROK's Yonhap News, the latest indication that the on-again-off-again summit between the DPRK and the US is going ahead.

Japanese public broadcaster NHK reported that a team of US government officials, including the White House deputy chief of staff for operations Joe Hagin, left US Yokota Air Base in Japan for Singapore on Monday.

It also reported that Kim Chang-son, Kim Jong-un's de facto chief of staff, flew to Singapore via Beijing on Monday night.

US President Donald Trump spoke over phone with Japanese Prime Minister Shinzo Abe on Monday, with the two agreeing to meet ahead of the expected meeting between the leaders of Washington and Pyongyang, said the White House.

During their phone conversation, the two leaders discussed recent developments for a possible meeting between Trump and the top leader of the Democratic People's Republic of Korea Kim Jong-un, according to a statement released by the White House.

Trump and Abe also agreed to meet again to "continue close coordination" ahead of the expected summit, which was originally scheduled on June 12 in Singapore, said the statement.

Trump on Sunday confirmed that a US official team had arrived in the DPRK to talk about the preparation for the summit.

Earlier, Trump canceled the scheduled meeting on Thursday, citing "tremendous anger and open hostility" displayed by Pyongyang recently. However, he reversed course one day after, saying both sides wanted the meeting to happen and it could still go ahead after productive talks.