"Our diplomacy has been in a state of vacuum due to the prolonged political impasse, but it appears (the envoys) have completely filled the gap with a single blow," Moon said in a meeting with his envoys to the three countries.

The special envoys include Hong Seok-hyun, a former ambassador to the US, who traveled to Washington last week for a meeting with US President Donald Trump and other ranking US officials.

Former Prime Minister Lee Hae-chan acted as Moon's special envoy to China, while Rep. Moon Hee-sang of the ruling Democratic Party visited Japan as the new South Korean leader's messenger there.

The president thanked the envoys for making themselves available on such short notice. And then he thanked them for accurately delivering his messages that needed to be said, especially those concerning thorny issues such as the deployment of the THAAD US missile defense system and a controversial deal with Japan over its sexual enslavement of Korean women during World War II.

Moon and his special envoy to Japan have told Japanese leaders, including Prime Minister Shinzo Abe, that South Korean people could not accept the 2015 agreement over sexual slavery.

Under the controversial deal, Tokyo agreed to provide 1 billion yen (US$8.94 million) in compensation to thousands, if not tens of thousands, of South Korean women forced into sexual servitude. The money is also intended for dozens of Korean victims who are still alive.

The UN Committee against Torture has also acknowledged the compensation may not be enough to help and support the victims in a recently published report.

"As well as the THAAD issue and the sexual slavery issue, I believe we have said what needed to be said," the president said.

"I need to hold summit talks with the leaders of those countries, and so I believe (such clear messages) have been meaningful also in the sense of setting the agenda for the summit talks," he added.

The South Korean leader came into office on May 10, only one day after a rare presidential by-election caused by the removal of the former president over corruption allegations.

Moon has already agreed to hold a bilateral summit with his US counterpart in Washington in late June.

He is also expected to hold summit talks with Chinese President Xi Jinping and the Japanese prime minister.

All four leaders, including Trump, are also set to attend the G20 summit scheduled to be held in Hamburg, Germany on July 7-8. (Yonhap)