A busy border crossing between North Korea and China demonstrates a close but fraught relationship between the two communist allies, the CBC’s Catherine Mercier reports in this video.

Dandong’s proximity to North Korea has made it the biggest trade hub between the two countries, and a bridge across the river is a lifeline for Pyongyang.

With crippling sanctions imposed on its banking system and little hard currency, North Korea sends trucks carrying output from its mines as barter for precious Chinese goods in exchange, keeping the bridge busy with traffic.

In February, after Pyongyang conducted a nuclear test, China took an unprecedented step and temporarily halted delivery of oil to North Korea, and there have been other signs of growing impatience with North Korea's belligerence.

Earlier this week, China’s new president, Xi Jinping, made a veiled reference to North Korea by saying no country should be able to "throw a region or even the whole world into chaos."

North Korea has become a burdensome ally for China, but the prospect of a regime collapse that could send a flood of refugees across the border and put South Korea and U.S. troops on its doorstep make China "pinch its nose and maintain its friendship with its unpredictable neighbour," says Mercier in her video report.