US President Donald Trump has accused China of being caught "red-handed" selling oil to North Korea, saying such moves would prevent "a friendly solution" to the crisis over Pyongyang's nuclear program.
The comments come after South Korean newspaper reported Chinese and North Korean vessels had been illicitly linking up at sea to get oil to North Korea.
"Caught RED HANDED - very disappointed that China is allowing oil to go into North Korea," Mr Trump wrote in a post on Twitter.
China earlier on Thursday denied there had been UN sanction-breaking oil sales by Chinese ships to North Korea.
Mr Trump also posted a video on Twitter comparing his comments on North Korea 18 years ago to those of former US President Bill Clinton 23 years ago.
In that video clip, Mr Trump described the North Korean regime as "sort of whacko", and urged action against its nuclear weapons program.
The Trump administration has led a drive to step up global sanctions on North Korea in response to Pyongyang's efforts to develop nuclear missiles capable of hitting the United States.
Washington says the full cooperation of China, North Korea's neighbour and main trading partner, is vital to the success of this effort, while warning that all options are on the table, including military ones, in dealing with North Korea.
The UN Security Council last week unanimously imposed new sanctions on North Korea for a recent intercontinental ballistic missile test, seeking to further limit its access to refined petroleum products and crude oil.
The UN resolution seeks to ban nearly 90 per cent of refined petroleum exports to North Korea by capping them at 500,000 barrels a year.
The US-drafted resolution also caps crude oil supplies to North Korea at 4 million barrels a year and commits the Council to further reductions if North Korea were to conduct another nuclear test or launch another ICBM.
In September, the Security Council put a cap of 2 million barrels a year on refined petroleum products exports to North Korea.
Spy satellites saw oil transfers, reports say
South Korea's Chosun Ilbo newspaper this week quoted South Korean government sources as saying that spy satellites had detected 30 instances of ship-to-ship transfers to North Korean vessels since October.
The newspaper said the transfers involved large Chinese and North Korean ships operating in waters between China and South Korea.
A State Department spokesman, Michael Cavey, reiterated on Wednesday that the United States had called on all countries to cut economic ties with North Korea.
"We urge China to end all economic ties with the DPRK, including tourism, and the provision of any oil or petroleum products, and expel all DPRK workers," he said, using the acronym for North Korea's official name, the Democratic People's Republic of Korea.
China has repeatedly said it is fully enforcing all resolutions against North Korea, despite suspicion in Washington, Seoul and Tokyo that loopholes still exist.
Asked at a regular briefing whether Chinese ships were illegally providing oil to North Korean ships, Chinese Defence Ministry spokesman Ren Guoqiang reiterated that China, including the military, strictly enforced UN resolutions.
Documents seen by Reuters this month showed Washington called on the Security Council to blacklist 10 ships for circumventing sanctions by conducting ship-to-ship transfers of refined petroleum products to North Korean vessels or transporting North Korean coal.
China and Russia subsequently asked for more time to consider the proposal.
- Reuters and ABC