South Korea has revealed it seized a Hong-Kong registered ship last month suspected of supplying oil to the North in breach of international sanctions.
Officials said the Lighthouse Winmore had secretly transferred 600 tons of refined oil to a North Korean ship.
The revelations came as China denied claims by President Trump that it had allowed oil shipments to the North.
A UN Security Council resolution bans ship-to ship transfers of any goods destined for Pyongyang.
On Thursday the US president tweeted he was “very disappointed” with China, which he said had been “caught red-handed”. He was responding to a South Korean report that suggested Chinese ships had been spotted selling oil to North Korean ships at sea in recent weeks.
The Chinese government swiftly rejected the president’s claim, saying it was “not consistent with the facts”.
“China has never allowed Chinese enterprises nor individuals to violate UN Security Council resolutions imposed on the DPRK,” foreign ministry spokesperson Hua Chunying said.
The Lighthouse Winmore entered Yeosu port in South Korea on 11 October to load up with refined oil and left bound for Taiwan, Yonhap news agency reported.
But instead of going to Taiwan it transferred the oil to a North Korean ship and three other vessels in international waters, South Korean officials were quoted as saying.
The Lighthouse Winmore remains in South Korea, the authorities told the BBC Korean service.
President’s Trump latest broadside against China came after South Korean newspaper Chosun Ilbo reported that Chinese tankers had been secretly transferring oil at sea to North Korean vessels.
Quoting South Korean government officials, it said the illegal ship-to-ship transfers had been filmed by US spy satellites about 30 times since October.
China, North Korea’s main trading partner, has repeatedly said it fully enforces all UN resolutions against Pyongyang.
Last week, Beijing supported a US-drafted UN resolution that included measures to slash the North’s petrol imports by up to 90%.
The tough new sanctions were a fresh attempt to curb Pyongyang’s controversial ballistic missile tests.
In another development on Thursday, the UN Security Council denied international port access to four more North Korean ships suspected of carrying banned goods, AFP news agency reported. It would bring the total number of ships blocked by the UN to eight.
North Korea is already subject to a raft of sanctions from the US, the UN and the EU.
The latest UN measures came in response to Pyongyang’s 28 November firing of a ballistic missile, which the US said was its highest yet.
In a typically bellicose response, North Korea described the new sanctions as an “act of war”.
Mr Trump has previously threatened to “totally destroy” North Korea if it launches a nuclear attack. North Korean leader Kim Jong-un has described the US president as “mentally deranged”.