SEOUL (REUTERS) - South Korean President Moon Jae In has been riding high after a string of successes with North Korea, seeing his popularity surge after his historic meeting with North Korean leader Kim Jong Un as he looks ahead to the US-North Korea summit he helped engineer next month (June).

But just days before a key meeting with US President Donald Trump, Pyongyang has made Mr Moon's role as a diplomatic go-between much harder, calling his government "ignorant and incompetent" and pulling out of inter-Korean talks.

Mr Moon heads to Washington next week with the daunting task of crafting a unified approach between the two allies ahead of the June 12 meeting between Mr Trump and Mr Kim in Singapore.

While publicly supporting Mr Trump's campaign of sanctions and military threats, Mr Moon has long called for reconciliation with the North and has pushed Washington to engage with Pyongyang's overtures.

North Korea's threat this week to scrap the Trump summit if Washington continues to press for unilateral denuclearisation and complaints about US-South Korea military drills has done little to advance Mr Moon's hope for a peace deal.

"Moon did an impressive job building up political capital with both leaders - with Trump throughout 2017 and Kim more in 2018 - and has spent some of it to create a situation that brings North Korea and the US to the table," said adjunct fellow Andray Abrahamian at the Griffith Asia Institute.

"He's still in a position to act as a go-between, but only as far as the other two parties take bridgeable positions."

The North's outburst aimed at Mr Moon's government appeared partially driven by anger over South Korea allowing Mr Thae Yong Ho, a former North Korean diplomat to Britain who defected to the South in 2016, to hold a press conference on Monday at the South Korean National Assembly for his publication of his memoir.

In his memoir, "Password from the Third Floor", Mr Thae describes Mr Kim as "impatient, impulsive and violent".

In rambling remarks in the White House's Oval Office, Mr Trump said that as far as he knew, the meeting with Mr Kim was still on track, but that the North Korean leader was possibly being influenced by Beijing after two recent visits he made there.

North Korea's renewed ties with China "certainly has emboldened" Mr Kim by making him less reliant on a successful summit with Mr Trump, but Beijing is unlikely to be directing the latest outbursts, said Mr Zhao Tong, a North Korea expert at the Carnegie-Tsinghua Centre in Beijing.

"I am sceptical about the speculation that China was behind Kim Jong Un's renewed hard line," Mr Zhao said. "I believe China wants North Korea and the US to reach a deal, which would make it easier for China to enhance economic cooperation with North Korea and better facilitate North Korea's strategic transition to economic development and open-up."

Mr Moon's administration quickly accepted Mr Kim's New Year's proposal to ease tensions, arranged a historic inter-Korean summit, and passed on Mr Kim's invitation to meet with Mr Trump.

North Korea, meanwhile, has pledged to publicly dismantle its nuclear test site next week.

South Korean officials said they still believe the nuclear site will be dismantled, and said they had not seen any changes in their lines of communication with the North Koreans.

Mr Moon is scheduled to meet with Mr Trump on Tuesday (May 22) in Washington.

Despite professed unity, Mr Trump has often taken a harder line on North Korea than Mr Moon, and the US president has repeatedly criticised South Korea over trade while questioning the usefulness of the alliance.

South Korea's presidential Blue House, however, says it hopes Mr Moon's meeting with Mr Trump will help set the stage for more progress with North Korea.

"As this US-South Korea summit is taking place three weeks ahead of the US-North Korea summit, we hope it will act as a bridge for the success of the US-North Korea summit," said second deputy director Nam Gwan Pyo of the Blue House's National Security Office.