North Korean economist Ri Gi Song, a researcher at the Institute of Economics at the Academy of Social Sciences, said the coal ban would not have a big effect.

"We have been exporting some good quantities of coal, but we didn't export very much," he said. "(Therefore) we don't feel very much affected by the so-called restrictions."

He said the export of other raw materials -- such as magnesite and graphite, used in smartphone production -- is "very important" for major world economies. North Korea sits on more than half of the world's deposit of those two raw materials, Ri said.

North Korea claimed success in its February 12 test of a new medium long-range ballistic missile, the Pukguksong-2. China voiced its opposition to the launch and joined other members of the UN Security Council in condemning Pyongyang's action.

When reports first surfaced last week that authorities in an eastern Chinese port rejected a large shipment of coal from North Korea, a Ministry of Foreign Affairs official declined to confirm the story but reiterated Beijing's long-held position.

"China fully, earnestly and faithfully enforces relevant Security Council resolutions, which include clear provisions on North Korea's coal exports," said ministry spokesman Geng Shuang last Wednesday.

Strained relations

In their first meeting, new US Secretary of State Rex Tillerson on Friday stressed to Chinese Foreign Minister Wang Yi the growing threat posed by North Korea's nuclear and missile programs.

Tillerson urged China to "use all available tools to moderate North Korea's destabilizing behavior," said acting US State Department spokesman Mark Toner.

While pledging to strictly implement UN-imposed sanctions, Wang told reporters in Germany, where both he and Tillerson were attending a G20 ministerial conference, that there is still hope to resolve the North Korea nuclear issue through diplomacy, and Beijing is willing to facilitate multilateral talks, according to a statement released by the Chinese foreign ministry.

In late January, several Chinese ministries jointly issued an extensive list of prohibited items for export to North Korea in compliance of Resolution 2321.

That ban covered military-civilian dual-use articles related to weapons of mass destruction -- including not only chemicals and weapons technologies, but also design software, high-speed cameras and truck chassis.

A foreign ministry official pointed to the list at the time as clear evidence to rebut skepticism over China's determination to implement UN sanctions against North Korea, its neighbor and longtime communist ally.

Beijing remains Pyongyang's biggest trade partner, providing an economic and political lifeline to the increasingly isolated regime.

China views North Korea as a strategic buffer between itself and South Korea, which has a sizable US military presence. It also fears a potential refugee crisis on its doorstep if the Pyongyang regime suddenly collapses.

Bilateral relations have become strained, however, since North Korea's young leader, Kim Jong Un, came to power after his father's death in late 2011.

The Chinese leadership is said to be growing frustrated with Kim's defiant stance on the nuclear issue and the resulting instability in Northeast Asia.