Friendship or leverage: Why is Xi Jinping going to North Korea?

For Chinese leader Xi Jinping, North Korea is the neighbour China can neither control nor afford to lose.

The two sides often describe their relationship as one "forged in blood", a reference to the Korean War.

Yet in recent years, mistrust has strained ties. And now Beijing is trying to reassert sway over a strategically vital yet deeply unpredictable partner.

China wants stability on its border and influence in Pyongyang, but without being dragged into crises triggered by North Korea's nuclear ambitions.

So Xi's visit this week is likely to be less about friendship, more about leverage.

Seoul believes he may try to position China as a mediator between North Korea and the United States, but Beijing may have other motives.

Western diplomatic sources tell the BBC that China has become increasingly concerned about the growing partnership between Pyongyang and Moscow.

After meeting Russian leader Vladimir Putin last week, Xi may want to ensure he also keeps North Korean leader Kim Jong Un in check, especially as Beijing increases its presence on the global stage.