BEIJING >> Trucks packed with seafood were backed up, bumper to bumper, at the Chinese border with North Korea. Protesters carried red banners demanding compensation. And Chinese businessmen who have been making big money from North Korean crabs, shrimp and squid were furious.

United Nations sanctions banning the import of North Korean seafood started to bite today, two days after China’s Commerce Ministry announced it would enforce the new rules passed by the U.N. Security Council as punishment for the North’s nuclear and missile tests.

The crackdown came as President Donald Trump offered some rare words of praise for North Korea’s leader, Kim Jong Un, making an apparent reference to the country’s decision to wait “a little more” before acting on plans to launch ballistic missiles toward Guam, a U.S. territory.

The Trump administration had been pushing China to tighten its enforcement of the U.N. sanctions, and North Korea’s export of seafood is a decent, if not spectacular, source of cash for its government.

By curtailing the trade, China, which has been criticized for not properly enforcing earlier sanctions, is obeying the intent of the latest sanctions resolution but harming its own businessmen.

“I think it is very likely that I need to return my truckload of seafood back to North Korea — and what’s worse, they won’t give my money back,” Zhang Xuebai, a wholesale trader, said in a telephone interview. “I will probably lose about $45,000. For other businessmen who have more goods stuck there, they can lose $150,000.”

The Chinese traders typically do not keep close track of international relations, and although they know that ties between China and its wayward ally, North Korea, can be touchy, they were not given any warning about the sanctions enforcement, Zhang said.

“This just happened out of the blue,” he said.

The red banners held aloft by protesters expressed the traders’ anger at the Chinese government.

“The precondition for sanctioning North Korea is that Chinese citizens should be protected from any loss,” one banner said.

“Hard-earned money is now stuck on the Chinese bridge. Chinese customs, please let it through,” said another.

This morning, the bridge between the North Korean and Chinese customs offices was jammed with trucks laden with seafood. A video was circulating that appeared to show the trucks lined up at the border.

By the end of the day, Zhang said, almost all of the trucks, including his own, were ordered to return to North Korea to return the seafood.

“I have a truck with 30 tons of frozen squid that is still stuck on that bridge,” said Chang An, a wholesaler based in Hunchun. “My truck has done the export paperwork with the North Korean customs. How am I going to return the goods? There is no way to return it, and I’m going to lose over $75,000.”

In all, North Korea earned $196 million from seafood exports last year, with almost all of that revenue coming from China, the Korea Trade Investment Promotion Agency said. Coal, North Korea’s most lucrative export, earned $1.2 billion, the agency said.

China stopped importing North Korean coal earlier this year, leaving seafood as one of the few remaining easy sources of revenue for the North. The sanctions passed by the Security Council this month prohibited the export of North Korean coal, iron ore, lead, lead ore and seafood. The ban on seafood, a highly visible export that goes almost directly to consumers, is probably one of the easiest to enforce.

North Korean shellfish has grown in popularity in China over the past few years. Big hotels and banquet centers buy the crab, shrimp and other delicacies because cheap North Korean labor makes for competitive prices. The fish is believed to be of high quality since it comes from relatively pure waters compared to the more polluted seas around industrialized Japan and South Korea.

The local government in Hunchun has attracted new seafood processing plants, boasting that it serves as the center of shipments of North Korean seafood to places beyond China, including the United States, Japan, South Korea and Europe. There have been reports of North Korean crab meat vacuum-packed in clear plastic being sold to Chinese traders who in turn dispatch it to the United States.

But the booming seafood trade in Hunchun is now on the verge of collapse, and wholesalers have closed their businesses, Zhang said. Further along the supply chain, restaurants in Beijing, Shanghai, Guangzhou and Shenzhen are now without their usual seafood supplies.

“The government has not yet offered any explanation to us,” he said.

The crackdown on North Korean seafood exports came as the highest-ranking U.S. military official held talks with his Chinese counterpart in Beijing.

Gen. Joseph F. Dunford Jr., the chairman of the Joint Chiefs of Staff, “delivered a clear message that North Korean ballistic missile and nuclear weapons threaten the entire global community” in a meeting on Tuesday with Gen. Fang Fenghui, according to an emailed summary of the gathering from a U.S. military spokesman, Capt. Darryn James.

Dunford emphasized to Fang “the need for China to increase pressure on the North Korean regime,” the summary said. If peaceful diplomatic and economic pressure failed to curtail North Korea, “Gen. Dunford reiterated America’s resolve to use the full range of military capabilities” to defend South Korea and Japan, as well as the United States.

© 2017 The New York Times Company