US President Donald Trump has said he's "very disappointed" with China following a report that it had allowed oil to be shipped into North Korea.
In a tweet, Mr Trump said China had been "caught red-handed".
He said there could never be "a friendly solution" to the North Korea crisis if oil was allowed to be exported to Pyongyang.
China earlier denied there had been any breaking of UN oil sanctions between China and North Korea.
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.
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.
US state department spokesman Michael Cavey reiterated calls for all countries to cut economic ties with North Korea, reports BBC.