BEIJING — “This is the empty bottle of my last North Korean beer. I sold it last night,” Cui Chengri lamented, speaking from his empty Beijing restaurant. “I need re-supply by tonight but I don’t know if there will be any.”

The United Nations has slapped stronger sanctions on North Korea over the last year because of the pariah state’s nuclear weapons and ballistic missile tests. That’s led to China, Pyongyang’s longtime ally, facing increased pressure to tighten its policies to support a unified global front.

China makes up more than 80 percent of North Korean trade and it has been credited with holding up the regime, but analysts say Beijing has become more assertive this year in pressuring its ally.

That pressure is playing out in myriad ways, including in Beijing restaurants.

“These beers were purchased a few months ago and I haven’t received new supplies since August when it became much harder,” said Cui.

The beer is in a standard dark green bottle that holds 500 milliliter. The label on the front says in Korean and English “Taedonggang Beer” and the back says it was produced in Pyongyang on June 21, 2017. Imported through Dandong, a Chinese border town, the beer journeyed a long way before it was consumed in this barbecue seafood restaurant five months later, at a price of 30 yuan ($4.54).

Restaurant boss Cui said he enjoys this beer himself, describing it as “sweeter” than rival brands.

Cui’s restaurant, opened in 2013, specializes in seafood — most of which is sourced from North Korea.

Born in China to ethnically Korean parents, Cui began his career as a travel agent for Chinese tourists in South Korea. Later, he met a seafood supplier and he turned the chance encounter into a business. On peak months in the first couple of years, the restaurant sold about five tons of (largely North Korean) shellfish and crabs.

But success didn’t last long. Direct daily seafood supplies were suddenly curtailed last February after North Korean missile launches and then they were halted completely after a new round of UN sanctions in August. Now, crabs have to travel through another country — Russia — before they arrive in China. As a consequence, prices doubled or even tripled and Cui lost 90 percent of his supply, he said.

Despite Beijing saying it would stop the sale of North Korean seafood in China, it’s not surprising that some is still entering the country through other channels,” said Michael Kovrig, senior advisor for North East Asia at the International Crisis Group, an independent analysis organization.

“Stopping that would require China to dedicate more resources to enforcement and investigation,” Kovrig said. “China is implementing sanctions relatively rigorously, but it could do more to monitor and enforce compliance.

“However, Beijing is probably worried that applying sanctions too strictly could destabilize North Korea and doesn’t want to take that risk,” he added.

—CNBC’s Xin En Lee contributed to this report.