Visitors walk across the Yalu River Broken Bridge, right, next to the Friendship Bridge connecting China and North Korea in Dandong in northeastern China's Liaoning province. (Emily Wang/AP)

The trucks still rumble across the Sino-Korean Friendship Bridge, and a nearby pipeline still pumps crude oil to keep the regime alive in Pyongyang.

But here in the Chinese city of Dandong, at the center of this country's trade with North Korea, pain and frustration are mounting.

Sanctions approved by the U.N. Security Council to punish North Korea for its nuclear and missile tests and bring it to the negotiating table are starting to bite.

"Personally, the sanctions are hurting me a tremendous amount," one Chinese trader said, explaining that almost 80 percent of the goods he used to move back and forth across the border — ranging from textiles to chemicals — are now forbidden.

"Both Chinese and North Korean business executives have the same thought — whatever happens, let it happen quickly," he said, requesting anonymity to speak on a sensitive subject. "If we have to have war, at least let it happen soon. We have to settle this quickly. Things can't go on like this."

Successive rounds of U.N. sanctions have cut off more than 90 percent of North Korea's publicly reported exports — including coal, iron ore, seafood and, most recently, textiles — and have restricted the regime's ability to earn foreign currency income by sending workers abroad.

China accounts for roughly 85 percent of North Korea's external trade and is seen by many as the key to forcing Pyongyang to at least freeze its nuclear and missile programs.

Along with Moscow, Beijing has balked at U.S. talk of a complete trade embargo. But foreign and Chinese experts said it has been implementing U.N. sanctions with unusual rigor and applying real pressure on provincial and local government officials in the border region.

The trader, of ethnic Korean descent, said he had not seen a worse climate in nearly two decades of doing business across the border. Nor had he seen the Chinese government so determined to impose its will — despite a lobbying effort from the local business community.

Customs checks have become more stringent at the border, traders said, causing delays in addition to making it harder to smuggle banned items.

In August, protests erupted near the northern end of the border, in the Chinese city of Hunchun, after Beijing moved to ban seafood imports from North Korea.

Lu Chao, a Korea expert at the Liaoning Academy of Social Sciences, said Chinese cold-storage plants and seafood processing workshops across the border have fallen idle, while companies involved in textiles, coal and iron ore are also suffering. Travel agents sending Chinese tourists to North Korea have seen their revenue affected.

"Sanctions bring a huge loss to Chinese traders," Lu said. "Many companies doing border trade have gone bankrupt, and their owners run away, leaving people unemployed."

In Dandong, a city that sits at the mouth of the Yalu River at the southern end of the border and that accounts for roughly 70 percent of cross-border trade, small groups of North Korean business executives are still easy to spot walking the streets and in hotels and restaurants near the bridge. Their plain and somber clothing, of dark blues, grays and greens, is livened only by the obligatory badge carrying the faces of the ruling Kim dynasty.

But outside the customs depot on the edge of town — where trucks are loaded and inspected before rolling across the border — two packers said traffic was falling.

On Yijing Street, where North Korean traders head to place their orders, their Chinese counterparts told a similar story. One woman who sells clothes to North Korea — "old-fashioned" colors, nothing bright, garments suitable for a cold country, she explained — has seen sales cut in half this year.

"China stopped buying their coal, and their money began to run out," she said.

Another woman said her business — sending textiles across the border for assembly and bringing them back as clothing for sale in China — was hurting badly, after Beijing banned the trade last week to comply with the latest U.N. resolution.

"We don't dare send anything across the border now," she said, "and we're trying to get our stuff back from there as quickly as possible."

So far, though, sanctions have had a relatively modest effect on the overall trade numbers. In August, China's imports from North Korea fell a modest 1 percent from a year ago, while exports were down 6.2 percent. On a cumulative basis, total trade was actually up 7.5 percent in the first eight months of the year.

That may partly reflect North Korea's ability to adapt to sanctions, as well as stockpiling by the regime and an attempt by traders to move goods across the border before sanctions hit, experts said.

Chinese exports of corn, bananas and rice rose sharply, and there was even a shipment of 1.8 million tons of coal in August — six months after a ban was supposedly implemented.

Commerce Ministry spokesman Gao Feng told reporters in Beijing that U.N. sanctions allowed a "cushioning" period after a ban is imposed, Reuters reported. Experts said China normally allowed shipments to go through if they had been agreed on before sanctions were announced.

But a bigger question perhaps is whether smugglers can bypass the sanctions.

In the seafood markets of Dandong, there are signs that they can — to some extent.

Water-filled tanks of crabs and clams dominate the offerings. With Chinese waters overfished, most would have come from North Korean waters in the Yellow Sea, one trader said. Another man, delivering crabs to the market from the back of his truck, complained that the ban had pushed up the price of the North Korean crustaceans by roughly 50 percent, which in turn had depressed demand.

"The formal trade from North Korea has stopped," a third trader said, "but there are still ways of getting it for people who are brave and willing to take the risk."

Fishing boats from Dandong used to cross to North Korea to pick up seafood, bringing it back to China to pass off as their own catch, bypassing customs.

Until recently, many did so openly during the day, but now the boats travel only at night, traders and fishermen said.

At the small but usually thriving Yicuomao port on a small channel of the Yalu River, scores of blue-painted wooden and steel fishing boats lay idle.

Some fishermen said they were losing money, but not everyone was.

"If you have connections, you can still go out," said one fisherman, unhappy at the unfairness of the situation. "On the surface, China is implementing sanctions, but it isn't really. If it really wanted to, it could just put a warship here to close off the estuary."

Experts said China can't be expected to completely shut down smuggling across the 880-mile border, any more than the United States can easily end illegal immigration from Mexico. But there are also reasons to doubt whether local officials are trying as hard as they might, said Christopher Green, senior adviser for the Korean Peninsula at the International Crisis Group.

While Chinese President Xi Jinping is reported to have disdain for his North Korean counterpart, Kim Jong Un, relationships at provincial and municipal levels, especially in security services, have always been very good. A little leakage serves to defuse domestic tensions and maintain that cross-border network, Green says.

Within North Korea, the economy is almost designed to withstand sanctions. When times are tough, trade will be even more strictly cornered by the military and security apparatus. The ban on textiles, meanwhile, will have a disproportionate effect on women in North Korea who assemble garments.

"The pain is felt at lower levels," Green said. "That is not to say sanctions are bad, but invariably the effects flow downhill at the end of the day."

Near the village of Xingguang on the outskirts of Dandong, paramilitary troops and a fire company guard 10 large oil storage tanks, the start of an underground pipeline that supplies North Korea with the crude it needs to keep its military and industry running. It is a symbol of Beijing's bottom line: keeping the Pyongyang regime alive.

North Korea has apparently been stockpiling fuel, experts said, one reason that civilian fuel prices there have risen significantly this year. It also means that the North could probably withstand a temporary cut in oil supplies.

But Beijing will never completely cut the regime's jugular vein, experts said. It fears that a cut in oil supplies could leave China facing a nightmare scenario: either a hostile and desperate nuclear-armed enemy or, if the regime collapsed, a refugee crisis followed by an American puppet state right on its border.

"Why do we sanction?" asked Lu at the Liaoning Academy of Social Sciences. "We believe that wrongdoings deserve punishment, but punishment should not be seizing them by the throat and trying to choke them to death."

Luna Lin contributed to this report.

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