North Korea has just conducted its fifth and biggest nuclear test yet. The international community seems powerless to stop the rogue nation and there is more pressure than ever for China to rein it in.

Key points: Trucks constantly travel from China to North Korea to deliver goods

Trucks constantly travel from China to North Korea to deliver goods There is little evidence customs agents are checking trucks as required

There is little evidence customs agents are checking trucks as required China is claimed to be using North Korea as a foreign policy tool

To the world, China says it backs and enforces tougher UN sanctions but on the ground it is a very different story. At the China-North Korea border the ABC has uncovered large-scale sanction violations that keeps the rogue regime going.

The Chinese city of Dandong is the centre for trade between North Korea and China. Its sits on one side of the Yalu River and is a bustling, thriving city where gleaming new skyscrapers rise up right to the river's edge and people enjoy new wealth. China provides up to 90 per cent of North Korea's trade and more than half comes from Dandong.

Just across the river, a couple of hundred metres away is North Korea and it is a stark contrast. Factories are at a standstill, ships lie abandoned and fishermen wearing raggy clothes dot the banks.

A North Korean stands guard at a jail on the banks of the Yalu River. ( Reuters: Jacky Chen )

In the middle of it all is the Friendship Bridge. It is North Korea's lifeline, with a constant steam of trucks delivering all types of goods across the border to prop up the rogue regime. The trucks are on a constant loop going back and forth.

A flying object soars into the air above North Korean territory as seen from Dandong (Feb 2016). ( Reuters: Kyodo )

In the customs area it is clear the tougher UN sanctions put in place after North Korea's fourth nuclear test are not working. Under the new sanction regime every truck is meant to be checked to ensure the goods are not destined for the elites of the regime, or the North Korean military and its nuclear program.

The ABC observed the area for three days and saw little evidence of tougher inspections. In fact, the most we ever saw was a token look at a truck from a customs agents. Most of the trucks are covered up with canvas but in some you can see all types of construction goods, car parts, tyres and pipes.

In one two-hour period the ABC counted about 100 trucks going from China into North Korea without being inspected. Anything could be going through.

Trade is booming and is now back to pre-sanction levels. The trucks are going 24-hours. Chinese authorities and businessmen exploit a "livelihood" loophole that is meant to minimise impact on North Korean civilians.

An increasingly blurred line

Tong Zhao from Beijing's Carnegie Tsinghua Centre for Global Policy says there is a lot of wiggle room in the new sanction regime and ultimately it's up to China to decide what could be dual use items.

"In principal there is no clear cut way to distinguish civilian and military uses of items – even food can be used to feed soldiers," he says.

"Any ordinary industrial machinery can be used to build trucks to transport systems, so there is no clear cut line."

He disputes what the ABC observed at the border and says China checks all trade.

A truck with relief supplies line up on the side of the Sino-North Korean border on April 26, 2006. ( AAP: Yonhap News )

The tougher sanctions have also led to an explosion in smuggling. The ABC met a Korean businessman who runs a series of restaurants and factories near the border. He wants to remain unidentified but says the black market accounts for 80 per cent of the overall trade.

"It's increased because relationship between China and North Korea is good and it happens all along the river," the Korean businessman says.

It is easy to see how that is possible. Security along the 1400-kilometre border is minimal – there's the odd North Korean soldier and the Chinese presence is small. The Korean businessman says the Chinese authorities and the locals have no intention of stopping the trade.

"Ordinary people want to trade to make money, they don't care about politics and North Korea has nothing – no food, no oil."

The ABC witnessed a smuggling operation underway on the Yalu river at about 10:00pm. It involved at least a dozen men. Boxes were unloaded, packed onto trucks and driven away. It was difficult to see what the contents were. We were told it was just one of many locations along the river where smuggling occurs.

A trader prepares Chinese goods to be loaded into trucks bound for North Korea. ( AFP: Mark Ralston )

In the city of Dandong spontaneous trade seems to pop up all over the place, making it difficult for authorities to control. One trader loading goods in the street told us: "the North Korean people come here to buy food for families. It's non-government trade, just for families".

Much of the economy in Dandong is built around trade, with North Korea and most experts agreeing China has no strategic interest in stopping it. They do not want North Korea to collapse because that could send a flood of refugees into China and bring US troops closer to the border.

Mr Zhao says China's policy is to engage with North Korea, not to isolate it, so it can bring North Korea back into the international community as a normal member.

But Mr Zhao also says China is supporting North Korea to use as its own foreign policy tool.

"US is trying to go after China and trying to contain China, like causing trouble in South China Sea, so China sees value in North Korea … North Korea can still cause trouble for the United States and it might divert attention away from China," Mr Zhao says.

Despite China saying it was "resolutely opposed" to the latest nuclear test, it is working towards a bigger and deeper economic partnership with North Korea. Just on the outskirts of Dandong a half billion-dollar bridge and a free trade zone has been built and is ready to service the increase.