China had not imported any coal from North Korea since Beijing imposed a cutoff on Feb. 19, Mr. Huang said. That ban has been interpreted as punishment for the February killing in Malaysia of Kim Jong-nam, the estranged half brother of North Korea’s leader, Kim Jong-un, which Pyongyang has been accused of orchestrating.

There have been other indications that China was following through on its promise to stop buying the North’s coal. Shipping data from this month showed a dozen cargo ships returning to North Korea fully laden, after China ordered its trading companies to return coal, Reuters recently reported. Mr. Trump referred to that as an encouraging sign in one of his interviews on Wednesday, a detail that appeared to show that he was paying attention to China’s actions against North Korea.

After the United Nations sanctions were announced, some economists said it was still possible for Chinese businesses to import coal on an off-the-books basis, using transactions that would not be recorded by customs officials.

But since mid-February, Chinese coal traders have said that their business has virtually vanished. “It’s over,” said a coal trader who operates from Dandong, a city on China’s northeastern border that functions as the main center of business with North Korea. The trader spoke on the condition of anonymity because he feared reprisals from the city authorities.

On Wednesday, Global Times, a prominent state-run Chinese newspaper, said that if the North tested another nuclear bomb or a missile, restrictions on oil might be the next step. North Korea is almost entirely reliant on China for crude oil.

“If the North makes another provocative move this month, the Chinese society will be willing to see the United Nations Security Council adopt severe restrictive measures that have never been seen before, such as restricting oil exports to the North,” the paper said in an editorial.

How much harder China would bear down on North Korea by its own volition remained an open question.

“It’s hard to ban normal trade that is not prohibited by U.N. resolutions,” said Yang Xiyu, a former Chinese diplomat who led China’s delegation to the so-called six-party talks on the North’s nuclear arms development in the mid-2000s.