Protests have broken out on the Chinese border with North Korea, as China's seafood importers watched frozen product melt in trucks blocked from crossing after China imposed a new round of economic sanctions.

Dozens of trucks have lined up on a bridge, unable to enter China, according to video shot by a local trader who posted it to Chinese social media. It shows a long line of unmoving truck containers stuffed with crabs and shrimp that were melting in the sunshine and risked becoming spoiled product in the latest sign that Beijing is, for now at least, strictly enforcing a new round of sanctions "There is nothing we can do but accept it as our fate," said Cui Yanzhi, a local trader in the Chinese city of Hunchun, where North Korean product has long underpinned a thriving business in the far northeastern corner of the country, not far from Vladivostok.

Now, he said, "a small city with a population of only 200,000, including tens of thousands working in the seafood industry, suddenly has nothing to do."

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The United States has often accused Beijing of doing too little to enforce sanctions against a country whose pursuit of nuclear-tipped long-range missiles has sent shivers of fear through capitals around the world.

But Pyongyang has angered Beijing through its belligerent defiance of international demands that it halt its weapons development program, and China was among the countries that signed off on a new round of United Nations economic sanctions in early August.

Roughly 90 per cent of foreign trade with North Korea moves through China, which imposed those sanctions starting at midnight Wednesday, immediately stopping iron, lead, coal and seafood from entering. Near the border city of Hunchun, some traders responded with protests, holding out banners demanding compensation for their losses.

"The money from our own blood and sweat is all sitting on a bridge to China. Please, Chinese customs, let us go," one banner pleaded.

The protest underscored the degree to which China is singularly responsible for the success of any international economic action against North Korea. "Owing to China's traditional economic ties with North Korea, it will mainly be China paying the price for implementing the resolution," Chinese Foreign Minister Wang Yi said last week.

It's not clear, however, that the blocked seafood signals a radical shift in policy in China, which has a long history of vigorous sanctions enforcement that it does not maintain for long.

"China views passing sanctions as a signal to send to North Korea, and what better way to send it than cutting off the trucks right at the border," said Justin Hastings, an international-relations scholar at the University of Sydney who wrote A Most Enterprising Country: North Korea in the Global Economy.

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"The question is whether they'll continue doing that over the long term."

There is reason to believe it won't happen. Previous rounds of sanctions have led to trucks blocked at the border, but that "probably lasts no more than a couple of months," Prof. Hastings said. "Then things go back to being the way they were."

That said, any spoiled seafood represents a clear loss to North Korea, and it's harder to stockpile food products for a later date, when scrutiny on sanctions might ease, than mined goods.

That means Pyongyang will feel an economic hit from the most recent sanctions, Prof. Hastings said: "I don't think it will be zero." But he doubts, too, that it will reach the $1-billion (U.S.) touted by the White House.

"Beijing is walking a fine line," said Thomas Eder, a research associate at the Mercator Institute for China Studies. "It is willing to accept relatively severe sanctions on paper, and inflict real damage on both the North Korean economy and its own traders. At the same time, it will not withdraw the economic lifeline to its neighbour."

Workarounds are likely. Take coal, where "China quickly stockpiled a year's worth before agreeing to an import ban," he said. "With seafood, other methods will likely be found in time."

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With seafood, in particular, the border is already porous, with large amounts of North Korean product smuggled across, or exchanged by fishing boats in international waters. Illegal logistics networks already in place provide an easy way to skirt sanctions.

Near Hunchun, the Chinese government gave warning that it would be closing the border to seafood, even keeping customs open until midnight Wednesday night. But problems arose when the North Korean side didn't let some traders exit the country in time. By Thursday, they were unable to enter China with their goods.

Some local sellers have already begun to adjust, said Huang Yujie, who runs Nanfang Seafood Shop in Hunchun with her husband.

"We are now selling Russian crabs," she said. The additional cost of bringing in Russian product, however, meant prices were rising. "The impact is certainly serious," she said. "There are quite a few shops that have stopped doing business because they have run out of supply."

Elsewhere, however, there were indications that companies didn't expect the ban on seafood imports to last.

Those with trucks stuck at the border were looking to get them back into North Korea, to deposit their contents in cold storage so they could wait for a better time, Mr. Cui said. He estimates that he still has $750,000 worth of seafood left inside North Korea.

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"We have to wait and watch," he said. "If it lasts too long, I will have to process it in North Korea and sell to their domestic market."

With files from Yu Mei