Trump said on Twitter that China had been "Caught RED HANDED," adding he was "very disappointed that China is allowing oil to go into North Korea."
"There will never be a friendly solution to the North Korea problem if this continues to happen!" the president said, without citing the source of his information.
China accounts for the vast majority of North Korea's external trade and oil supplies. Just two weeks ago, Trump credited China for its help in the US-led pressure campaign against Pyongyang, including support for three UN Security Council resolutions this year -- the latest last week -- over the North's nuclear and missile tests.
China's foreign ministry has defended its enforcement of UN sanctions against North Korea. A ministry spokeswoman said Wednesday she had no information about the latest report, but said China has strictly enforced trade restrictions.
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The State Department did not immediately respond to a request for comment on that report. But in a commentary Thursday, Secretary of State Rex Tillerson urged Beijing to exert "decisive economic leverage" on Pyongyang.
Ship-to-ship trade with North Korea at sea is prohibited under UN sanctions adopted on September 11. The latest sanctions adopted on Friday, in response to the test of a North Korean intercontinental ballistic missile, also impose sharp reductions on imports of refined oil products by the isolated nation.
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.
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.
On Decemeber 15, Trump said Russia, another significant trading partner with North Korea, had failed to help, while China had.
"China is helping. Russia is not helping.We'd like to have Russia's help -- very important," Trump told reporters after a phone conversation with President Vladimir Putin.
In the interview, Trump said he'd be willing to launch a pre-emptive strike against North Korea if negotiating "like crazy" didn't work. And he describes the country as "sort of wacko.