Trump hits out at China giving oil to NK

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

Trump said on Twitter that China had been "caught" allowing oil into North Korea and that would prevent "a friendly solution" to the crisis over North Korea's nuclear program.

"I have been soft on China because the only thing more important to me than trade is war," Trump said in a separate interview with The New York Times.

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

US 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 October 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 UN Security Council resolutions pertaining to North Korea in their entirety and fulfils 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 UN 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.

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.

South Korea said on Friday it had seized a Hong Kong-flagged ship suspected of transferring oil to North Korea in defiance of the sanctions.

A senior South Korean foreign ministry official said the ship, the Lighthouse Winmore, was seized when it arrived at a South Korean port in late November.