China on Friday denied reports it has been illicitly selling oil products to North Korea, after U.S. President Donald Trump said he was not happy that China had allowed oil to reach the isolated nation.

Trump said on Twitter the previous day that China had been "caught" allowing oil into North Korea and that would prevent "a friendly solution" to

"I have been soft on China because the only thing more important to me than trade is war," Trump said .

South Korea's Chosun Ilbo newspaper this week quoted South Korean government sources as saying that U.S. spy satellites had detected Chinese ships transferring oil to North Korean vessels about 30 times since October.

U.S. officials have not confirmed details of this report. Chinese Foreign Ministry spokeswoman Hua Chunying told reporters she had noted recent media reports including suggestions a Chinese vessel was suspected of transporting oil to a North Korean vessel on Oct. 19.

"The Chinese side has conducted immediate investigation. In reality, the ship in question has, since August, not docked at a Chinese port and there is no record of it entering or leaving a Chinese port," Hua said.

She said she was not aware if the vessel had docked at the port in other countries but the relevant media reports "did not accord with facts".

"China has always implemented U.N. Security Council resolutions pertaining to North Korea in their entirety and fulfills its international obligations. We never allow Chinese companies and citizens to violate the resolutions," Hua said.

"If, through investigation, it's confirmed there are violations of the U.N. Security Council resolutions, China will deal with them seriously in accordance with laws and regulations."

In the New York Times interview, Trump explicitly tied his administration's trade policy with China to its perceived cooperation in resolving the North Korea nuclear crisis.

"When I campaigned, I was very tough on China in terms of trade. They made - last year, we had a trade deficit with China of $350 billion, minimum. That doesn't include the theft of intellectual property, O.K., which is another $300 billion," Trump said, according to a transcript of the interview.

"If they're helping me with North Korea, I can look at trade a little bit differently, at least for a period of time. And that's what I've been doing. But when oil is going in, I'm not happy about that."

An official of the U.S. State Department said the U.S. government was aware of vessels engaged in such activity involving refined petroleum and coal.

"We have evidence that some of the vessels engaged in these activities are owned by companies in several countries, including China," the official said, speaking on condition of anonymity.

The United States says the full cooperation of China, North Korea's neighbour and main trading partner, is vital to the success of efforts to rein in North Korea, while warning that all options are on the table, including military ones, in dealing with it.

China has repeatedly said it is fully enforcing all resolutions against North Korea, despite suspicion in Washington, Seoul and Tokyo that loopholes still exist.