SEOUL (BLOOMBERG) - South Korea disclosed a probe into a local ship suspected of illegal high-seas cargo transfers with North Korean vessels, affirming support for US-backed sanctions ahead of a summit with President Donald Trump.

Authorities impounded the South Korean-flagged oil products tanker P Pioneer in the southern port of Busan in October, a government official said on Thursday (April 4), confirming an earlier report in the Chosun Ilbo newspaper.

The ship - the first local vessel seized by South Korean authorities - is among four detained for potential violations of United Nations curbs on fuel shipments to North Korea, Chosun said.

The disclosure comes as South Korean President Moon Jae-in, who sought to serve as a bridge between Mr Trump and Mr Kim, prepares to travel to Washington next week to resuscitate nuclear talks.

Mr Moon has faced attacks from both sides since the US-North Korea summit in Hanoi collapsed without a deal, with opposition lawmakers accusing his government of going soft on the regime and North Korea criticising it for supporting US-backed sanctions.

The economic penalties are expected to be a main topic of discussion when Mr Moon meets with the US President at the White House on April 11.

The talks between Mr Kim and Mr Trump broke down over disputes concerning how much sanctions relief was warranted for the disarmament steps offered by North Korea.

'MEANINGFUL' TALKS

Mr Moon's nuclear envoy, Mr Lee Do-Hoon, told a forum in Seoul on Thursday that more "meaningful" working-level talks, rather than tougher penalties, were necessary to resolve the dispute.

"To believe that the strongest sanctions and more pressure alone would make North Korea suddenly give up this entire nuclear programme is an illusion," Mr Lee said.

North Korea has successfully evaded sanctions to import as much as seven-and-a-half times the allowed amount of refined petroleum last year, with the ship-to-ship transfers playing a large role in providing fuel and materials to the energy-starved regime, according to the US government.

North Korean ports received at least 263 tanker deliveries of refined petroleum through ship-to-ship transfers last year, according to data from the State and Treasury departments and the US Coast Guard.

If the tankers were fully laden, North Korea would have imported 3.78 million barrels of fuel - less than a tenth of what South Korea imports in a month.

"Ship-to-ship transfers involve increasingly advanced evasion techniques," a UN Security Council report said in March.

The vessels disguise their identities, turn off transponders and often conduct transfers under the cover of darkness to avoid detection.

CLANDESTINE TRIPS

Such transfers of petroleum products by foreign-flagged vessels have increased in scope, scale and sophistication, with more than 50 vessels and 160 associated companies under investigation, the UN report said.

The vessels use social media to communicate and alter documents to dupe global banks and insurance companies, it said.

The P Pioneer was being held on suspicion of providing about 4,320 tonnes of diesel to two North Korean vessels via transfers in international waters in the East China Sea in September 2017, Chosun said.

Ship-tracking data compiled by Bloomberg shows the vessel making a trip matching that description during the same month.

The owner listed in maritime records for the P Pioneer, Daon Logistics Co Ltd, didn't immediately respond to a request for comment on Thursday.

Mr Moon has seen his approval rating slip since the summit, hitting a record low of 43 per cent in a Gallup Korea poll released last week.

Mr Moon has attempted to preserve his role as mediator between the two sides, praising Mr Kim's offer to dismantle a key nuclear production complex as an "irreversible" disarmament step, breaking with the Trump administration's characterisation of the offer.