BEIJING (BLOOMBERG) - In the fishing grounds where the Yalu River opens up to the Yellow Sea, Chinese and North Korean trawlers intermingle as they search for crabs, conch and yellow clams.

Drifting among them are Chinese boats called "mother ships" that act as floating middlemen, offering dollars, renminbi and even goods like cigarettes for the latest catch, according to traders who have been aboard the vessels.

One of them, who called himself Mr Du, said the seafood is then taken ashore to China and sold in wholesale markets, where it all gets mixed together.

The practice is just one form of smuggling along China's 1,350km border with North Korea, roughly the distance from Paris to Rome. Locals use boats, cars, trucks and several rail lines to carry everything from diesel fuel to silkworms to mobile phones back and forth across the Yalu.

The informal border trade shows the difficulties that authorities in Beijing face in fully implementing United Nations sanctions against North Korean leader Kim Jong Un's regime. As North Korea's biggest trading partner, China has come under attack from US President Donald Trump for preserving an economic lifeline for Mr Kim while he pushes for the ability to strike the United States with a nuclear weapon.

On Friday (Sept 15), North Korea fired another ballistic missile over Japan.

For China, implementing sanctions is a tricky balance. It wants North Korea to stop doing anything that leads the US to bolster regional defences that could also be used against China. At the same time, authorities have long feared that a collapse of the regime in Pyongyang could destabilise China's northeastern region and bring US troops to the banks of the Yalu.

This week, China voted in favour of UN Security Council sanctions to ban North Korean exports of textiles, limit the amount of oil and refined petroleum products sold to the country and strengthen inspections of ships suspected to contain prohibited items.

Together with previous sanctions on goods such as coal, iron and seafood, the US said about 90 per cent of North Korea's exports last year are now off limits.

While the latest penalties will take effect from Oct 1, a ban on North Korean seafood passed a month ago - taking away roughly US$300 million (S$404 million) in revenue each year - came fully into force only on Sept 5.

Interviews along the border last week with dozens of traders, wholesalers, smugglers, former local officials and foreign diplomats showed that fresh North Korean seafood was still available even as China visibly stepped up enforcement.

China's border with North Korea stretches from the industrial town of Dandong north to the town of Hunchun, near where the countries converge with Russia. Along the route, police and military have increased patrols and set up checkpoints to inspect vehicles.

Foreign affairs offices for the Dandong and Hunchun city governments did not respond to faxes seeking comment on efforts to stop smuggling.

China's Commerce Ministry spokesman Gao Feng told reporters in Beijing on Thursday that China will strictly implement the new sanctions.

In Hunchun, dozens of seafood wholesalers had closed after the earlier sanctions took effect. Chinese authorities seized shipments of North Korean squid at the border, according to shopkeeper Shi Haiyan at Quanhe Port, which sits on a river linked with the Sea of Japan/East Sea.

"The sanctions are strict now - seafood can't come through at all," the 34-year-old said last week.

Even so, restaurants in Hunchun were still selling North Korean crabs and conch. The goods are harder to find but still available, according to a shop owner who asked to be identified only by his surname Lyu.

The situation was similar in Dandong, the biggest Chinese city along the border and the centre of the country's trade with Mr Kim's regime. Dandong is home to a pipeline that regularly supplies oil to North Korea - a crucial supply source that was exempted from the new sanctions.

The city of 2.4 million people has several bridges that cross the Yalu, one of which is inoperable because North Korea hasn't built a road linking to it. Another one bombed out during the Korean War attracted Chinese tourists singing Communist songs about defeating America.

Hundreds of cars and trucks traverse the main Friendship Bridge each day, including many North Korean drivers looking to fill up with petrol. Getting across has become harder after the latest round of sanctions came into effect, according to Mr Wang Lisheng, 64, a former county official from nearby Hekou village who used to trade metals with North Korea.

At Dong Sheng, Dandong's main seafood market, four traders said last week they could still source the city's signature yellow clams from North Korea even though supplies had dropped. Mr Ha Wei, 38, said the price of dried clams had risen 20 per cent to 30 yuan (S$6) per half kilogram since the sanctions took effect.

About 40km away at the Yellow Sea Seafood Products Market, a larger complex where hundreds of workers sift through freshly unloaded seafood that is then shipped throughout China, multiple traders told Bloomberg they also were still able to procure goods from North Korea.

"We still have North Korean goods but much less in the last week after sanctions," said conch trader Xu E, 44.

Mr Du, who described how smugglers bring North Korean seafood into China, has been running goods across the China-North Korea border for the past 20 years. He has been detained in North Korea several times, including once when he was fed only carrots for three days before being released.

In the 1990s, he said, border smugglers regularly dealt everything from coal to diesel to North Korean brides. He avoided trading guns, drugs or people - things that could earn him a prison sentence instead of a fine. Despite the risk of violating sanctions, he said, the easy money will continue to attract smugglers on the border.

"As long as there's demand, smugglers will keep coming," Mr Du said. "No matter how hard Beijing tries."