But the alliance is not without tension.

Ties were frosty for much of the 2010s as Beijing’s patience over Pyongyang’s nuclear weapons programme wore thin. Kim Jong Un conducted a nuclear test in 2017 and has since ramped up tests of banned ballistic missiles – all of which increased US wariness, and strengthened its military alliances with South Korea and Japan.

China’s attitude has shifted more recently.

“Whilst only a decade ago, Beijing was overtly critical of North Korea’s nuclearisation – and indeed calling for Pyongyang’s denuclearisation – China is now saying little, if anything, to this effect,” Edward Howell, a lecturer at the University of Oxford, tells the BBC.

Observers say one reason is that North Korea has grown closer to Russia because of the Ukraine war.

At least 2,300 North Korean soldiers are believed to have died fighting for Russia in Ukraine. In return, Russia supplies North Korea with oil, in violation of UN sanctions. In 2024, Russia was estimated to have sent more than a billion barrels of oil as payment for Pyongyang’s weapons and troops.

Last month, North Korea and Russia opened their first road bridge, which Russian state media said would allow the movement of up to 300 vehicles and more than 2,000 people every day.

So Beijing is believed to be trying to reassert its sway over Pyongyang by rekindling ties.

Air China resumed direct flights between Beijing and Pyongyang in March after a six-year hiatus triggered by the pandemic. The two countries reopened passenger train services the same month.

Chinese President Xi Jinping’s first state visit of this year, in June, was to Pyongyang, where he met North Korean leader Kim Jong Un.

“The opening of the New Yalu River Bridge demonstrates bilateral relations are improving and that Xi Jinping’s June visit to North Korea was more than symbolic,” says Leif-Eric Easley, a professor at Ewha University in Seoul.

“But the long delay, including North Korea’s pandemic self-isolation and domestic socioeconomic crackdowns, shows what a difficult partner Pyongyang can be for Beijing.”