Smuggling is rampant. The export of North Korean rare earth minerals and gold, banned under the new rule, is one of the more lucrative revenue sources for the North Korean government, traders said. That business continues on privately owned 200-ton ships belonging to Chinese smugglers based here, they said.

The United Nations rules put the onus on customs inspectors here to judge which goods may help the nuclear program or the military, which are banned, and which are intended for civilians, which are allowed.

On a recent day, the customs checkpoint, a large outdoor parking area adjacent to the bridge, held a collection of China’s castoffs: cheap four-wheel-drive Haval passenger vehicles, discount medicines for hepatitis and tuberculosis, old solar panels to brighten dark houses.

But the customs office here lacks the staff to open all the containers, a local government official said. Like most people interviewed for this article, he spoke on condition of anonymity since there are risks to speaking candidly to foreign media about trade with North Korea.

At peak times, up to 200 trucks a day cross the Yalu River to Sinuiju, North Korea. Before departing, only about 5 percent of the containers they carry are inspected, the official said.

China and the United States worked closely on the new sanctions, and both countries said ridding North Korea of nuclear weapons was a shared concern. When President Obama and the Chinese leader, Xi Jinping, meet in Washington on Thursday, Mr. Obama is likely to press China to enforce the new measures.