WASHINGTON – President Trump said he was "very disappointed" over a report that China is continuing to supply oil to North Korea – as pressuring the rogue nation to give up its nuclear weapons program is one of the president's biggest challenges heading into 2018.

"Caught RED HANDED - very disappointed that China is allowing oil to go into North Korea," Trump tweeted.

He added: "There will never be a friendly solution to the North Korea problem if this continues to happen!"

The president appeared to be reacting to a South Korea newspaper report that said Chinese and North Korean vessels have been meeting at sea for transfers of oil. China denies it is violating United Nations sanctions on oil sales to Kim's government.

The tweet underscores one of the most vexing challenges facing Trump next year: What to do about North Korea, which says it is developing nuclear-tipped missiles that can reach the United States mainland.

More:South Korea predicts U.S.-North Korea talks will take place in 2018

More:North Korea says it's a 'pipe dream' that it will give up nukes

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.

Trump has spent months lobbying China and other nations to cut off economic assistance to Kim Jong Un's government. That was the main topic of the president's high-profile trip to Asia in November, including a visit with Chinese President Xi Jinping.

Since then,Trump has praised China for its cooperation – but not so on Thursday.

North Korea, meanwhile, says Trump's hopes that it will give up its nuclear weapons is a "pipe dream."