On Tuesday, the South Korean newspaper Chosun Ilbo claimed U.S. surveillance satellites spotted Chinese vessels engaged in ship-to-ship oil transfers with North Korean ships at sea, citing government sources in Seoul.
US President Donald Trump - who has often hailed China's efforts to put pressure on North Korea - hit out at Beijing Thursday for failing to cut off Pyongyang's oil supply, saying such moves prevented a "friendly solution" of the crisis.
A senior administration official told CNN that the Trump administration is planning to be "more quiet" and "more discreet" about speaking publicly on USA military exercises with the Republic of Korea and Japan aimed at demonstrating military capability against North Korea.
China has commented on reports alleging that it exports oil to North Korean by saying that Beijing " completely and strictly" implements United Nations trade restrictions recently adopted against Pyongyang. That report was picked up by some USA media outlets, including Fox News.
"Caught RED HANDED - very disappointed that China is allowing oil to go into North Korea", Trump said on Twitter.
Members of the Trump administration have repeatedly vowed to stop North Korea's nuclear program - and have not ruled out a military option that some analysts fear could lead to a nuclear conflict. Last week, the U.N. Security Council imposed new caps on trade with North Korea, including limiting oil product shipments to just 500,000 barrels a year.
Such ship-to-ship trades are banned under a UN Security Council resolution adopted in September, but according to South Korean government sources, American satellites have pictured large vessels from both China and North Korea illegally trading in a stretch of the West Sea on multiple occasions.
The State Department did not immediately respond to a request for comment on that report.
Chinese foreign ministry spokeswoman Hua Chunying said she had no information following Chosun's report, but said "the Chinese government has been completely and strictly enforcing Security Council resolutions" aimed at discouraging North Korea from developing nuclear and missile technology. The US Treasury Department later placed six North Korean shipping and trading companies and 20 of their vessels on sanctions list.