Beijing reaffirmed its opposition to "long-arm jurisdiction" after Washington imposed sanctions on Tuesday against three Chinese companies and a Chinese individual for engaging in trade with Pyongyang.

"China has always strictly followed and implemented all United Nations Security Council resolutions, and our efforts are obvious to all," Foreign Ministry spokesman Lu Kang said on Wednesday.

In all, one person, 13 trading entities and 20 vessels from various countries were sanctioned by the US Treasury Department, it said on Tuesday. They are accused of engaging in trade with the Democratic People's Republic of Korea worth hundreds of millions of dollars, it said.

"We are opposed to a certain country's wrongdoing in carrying out unilateral sanctions and long-arm jurisdiction in accordance with its own domestic laws," Lu told a daily news briefing in Beijing, adding that the United States is clear about China's position.

US officials announced the action a day after US President Donald Trump put the DPRK back on a list of state sponsors of terrorism.

Lu also said China will look into citizens and entities that conduct activities in Chinese territory violating China's domestic laws as well as China's international obligations.

"If other countries do have information and are willing to cooperate with China, they could definitely share the information with us and appropriately solve the issue," he said.

Da Zhigang, director of the Institute of Northeast Asian Studies at the Heilongjiang Academy of Social Sciences, said, "This is contradictory to the signal that Washington sent out earlier, and will not help with cooperation on this issue."

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