But China's Foreign Ministry said its government had fully implemented U.N. Security Council resolutions on North Korea and would punish anyone caught violating the Security Council sanctions under Chinese law.
It added that it opposed sanctions outside the framework of the Security Council.
"China especially opposes any country conducting 'long-arm jurisdiction' over Chinese entities and individuals," spokeswoman Hua Chunying said at a routine news conference. "Measures taken by the United States are not helpful in solving the problem and unhelpful to mutual trust and cooperation. We ask the United States to stop the relevant wrong practices immediately."
Despite China's professed opposition to unilateral sanctions, however, it has not hesitated to punish countries through trade if they refuse to do Beijing's bidding.
China is engaged in a major blockade of South Korean companies because it opposes the deployment of a U.S. missile defense system, known as Terminal High Altitude Area Defense (THAAD), in South Korea.
Among the sanctioned Chinese companies is Dandong Zhicheng Metallic Material, also known as Dandong Chengtai, one of the largest importers of North Korean coal, while its main shareholder also was individually targeted.
In a related complaint the Justice Department filed Tuesday, the U.S. government is seeking $4 million from the company, accusing it of importing North Korean coal and sending an array of products — cellphones, luxury items, rubber and sugar — to North Korea.
In August, the Security Council agreed to a total ban on coal imports from North Korea; in the past, a limited trade had been allowed, provided the coal was not purchased from a sanctioned North Korean company and was for "livelihood purposes."
In practice, experts say, that loophole was exploited to facilitate a trade that generated $1 billion a year for North Korea.
In a June report by the Washington-based research group C4ADS, Dandong Zhicheng was cited among those Chinese companies that are pivotal to North Korea's ability to circumvent international sanctions and buy illicit goods. The report said targeting those companies could cause North Korea's entire overseas network to collapse.
Dandong Zhicheng alone accounted for 9.2 percent of North Korea's total exports to China last year, according to documentation that C4ADS reviewed. Almost all — 97 percent — of this was North Korean coal, totaling about $250 million annually.
The company's website says it does a "small amount of business" in North Korean border trade.
Chi Yupeng, the company's main shareholder, won an award from the Dandong city government as a leading entrepreneur in 2005, another award for his "remarkable contribution to enterprise" in 2008 and a third for "starting a foreign trade enterprise" in 2009.
Two other Chinese companies, Dandong Tianfu Trade and Jinhou International Holdings, were sanctioned for their part in the coal trade. Together, the three companies imported nearly $500 million worth of North Korean coal between 2013 and 2016, the Treasury Department said.
Dandong Rich Earth Trading was sanctioned for buying vanadium ore from a company connected to North Korea's atomic energy agency, while Mingzheng International Trading was accused of facilitating dollar transactions on behalf of North Korea's proliferation network. The companies either declined to comment, hung up or did not pick up the phone when contacted by The Washington Post. The Dandong city government declined to comment.
The Global Times, a nationalist tabloid, argued that the United States could face retaliation if it continued to impose sanctions considered a "serious violation of international law" and "certainly unacceptable" to China.
"As far as we are concerned, Washington wants to use such unilateral sanctions to smear China and Russia's international image on issues regarding sanctioning North Korea, painting China and Russia as the destroyers of U.N. sanctions," it wrote.
Chen Weihua, deputy editor of the U.S. edition of China Daily, argued that the U.S. decision would undermine cooperation between Washington and Beijing.
"The U.S. has long believed that sanctions are a silver bullet," he wrote in a column. "But its past track records have shown that the majority of sanctions not only failed but caused humanitarian disasters in other countries."
There is little doubt, he added, that such secondary sanctions will have little or no effect in persuading North Korea to change course.
Beijing is not averse to using economic pressure to get its way: It significantly cut salmon imports from Norway for years after imprisoned democracy activist Liu Xiabo was awarded the Nobel Peace Prize, has encouraged consumer boycotts of Japan, and punished countries for hosting the Dalai Lama, the exiled Tibetan spiritual leader.
Nevertheless, Lu Chao, a Korean Peninsula expert at the Liaoning Academy of Social Sciences, said Washington's decision to impose unilateral sanctions on China was inappropriate, unacceptable and "very domineering."
"China is strictly implementing the U.N. resolution. Though we can't 100 percent exclude the possibility of individuals having underground deals violating the Ministry of Commerce's regulations, the United States being so far away doesn't have a proper reason to take sanction measures unilaterally," he said. "They should inform the Chinese government and let the latter deal with it."
China accounts for about 90 percent of North Korean trade but announced this month that it was suspending imports of iron ore, iron, lead, coal and seafood products from North Korea, to comply with U.N. sanctions.
China's imports from North Korea fell to $880 million in the first six months of the year, down 13 percent from a year earlier, official figures show. But Chinese exports rose 29 percent, to $1.67 billion, in the first six months of the year, increasing total trade between the two countries by 10 percent.
China is reluctant to do anything that might destabilize North Korea, a long-standing ally. It says U.S. hostility toward Pyongyang forced the regime to develop its nuclear program.
Shirley Feng and Luna Lin contributed to this report.