Trump asked Chinese President Xi Jinping in November to cut off North Korea's oil supply entirely, the American ambassador to the U.N., Nikki R. Haley, said at the time. Trump wrote in a post on Twitter.
USA spy satellites have captured images of what appears to be Chinese ships illegally selling oil to North Korean boats.
Trump has blown hot and cold on China's efforts on North Korea, which despite a dramatic increase in sanctions has made major progress this year on perfecting a nuclear-tipped missile that could strike the United States.
Washington says the full cooperation of China, North Korea's neighbor 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.
Last week, the Security Council unanimously approved tighter sanctions on North Korea, including measures to slash imports of refined petroleum products by nearly 90 percent from January, as well as further restrict shipping and imposing a 24-month deadline for expatriate North Korean workers to be sent home.
The U.N. resolution seeks to ban almost 90 percent of refined petroleum exports to North Korea by capping them at 500,000 barrels a year.
The US-drafted resolution bans the supply of almost 75 per cent of refined oil products to North Korea, puts a cap on crude deliveries and orders North Korean nationals working overseas to be sent back by the end of 2019.
A White House official and multiple National Security Council officials were unable to explain the President's tweet and did not respond to questions about whether he was referring to the recent reports from South 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.
However, on Tuesday the South Korean newspaper Chosun Ilbo cited unidentified South Korean government officials as saying USA reconnaissance satellites have spotted Chinese ships transferring oil to North Korean vessels some 30 times since October in seas off China.
Satellite images released by the U.S. Department of Treasury appeared to show vessels from both countries illegally trading oil in the West Sea, The Chosun Ilbo reported Tuesday, citing South Korean government sources.
Last month, the Treasury Department sanctioned six North Korean shipping and trading companies and 20 of their vessels, and published photos of what it said was a North Korean vessel on October 19 possibly transferring oil to evade sanctions.
"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.
"With President Trump's latest Tweet it seems the "Bromance" between him and President Xi is finally over", he said.
The US Treasury Department announced in November that a series of North Korea shipping and trading companies had attempted to conduct a ship-to-ship transfer - likely of oil - to evade United Nations sanctions.
"The solution is a coherent strategy, not bluster", he said.
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