The U.S. Treasury announced the action one day after President Donald Trump put North Korea back on a list of state sponsors of terrorism, on its website.

The new curbs show the Trump administration’s focus on hurting trade between China and North Korea, which it sees as key to deterring Pyongyang from its ambition to develop a nuclear-tipped missile capable of hitting the United States.

“This designation will impose further sanctions and penalties on North Korea and related persons, and supports our maximum pressure campaign to isolate the murderous regime,” said Treasury Secretary Steven T. Mnuchin.

The latest sanctions included blacklisting three Chinese companies, Dandong Kehua Economy & Trade Co., Dandong Xianghe Trading Co., and Dandong Hongda Trade Co., which the Treasury Department said had done more than $750 million in combined trade with North Korea over almost five years until Aug. 31.

It said they were involved in trade of coal, iron ore, lead, zinc and silver ore, lead metal and ferrous products as well as notebook computers.

The sanctions also blacklisted Sun Sidong and his company Dandong Dongyuan Industrial Co. In a June report, Washington think tank C4ADS said the firm was part of an interconnected network of Chinese companies that account for a vast share of trade with North Korea.

U.S. authorities have repeatedly targeted companies and individuals from the Chinese city of Dandong, which borders North Korea, in a bid to cut off Pyongyang’s major export revenue from selling natural resources, such as coal.

The direct impact on listed companies may be limited as trade between China and its isolated northern neighbour has slowed substantially since the United Nations imposed a ban in September on North Korean exports of coal, iron, iron ore, lead, lead ore and seafood.

In February, China also prohibited coal purchases from North Korea.

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China does not strictly enforce financial rules in the Dandong area, said Anthony Ruggiero, a North Korea expert at the Foundation for Defense of Democracies.

As a result, Dandong draws companies interested in making a profit by selling to North Korea, he said.

The new sanctions also hit several North Korean companies that send workers to countries such as Russia, Poland, Cambodia and China. United States authorities said they are seeking to cut off the money North Korea makes from the export of labor.

Besides targeting sources of weapons technology, the curbs were the first time the United States sought to directly attack North Korea’s daily consumer trade, said Peter Harrell, a sanctions expert at the Center for a New American Security.

“We are sanctioning companies involved in ordinary trade,” Harrell said. “That’s the logical next step.”

The sanctions aim to further isolate Pyongyang, said Heather Nauert, a State Department spokeswoman, adding that she did not think targeting more Chinese firms would affect Beijing’s cooperation on the North Korean issue.

“I don’t think it jeopardizes anything,” she said. “I think the world has come together on this issue. “We have a good relationship with China. That’s not going to change.”