Battle lines Pyongyang's Russia entente is a dilemma for Xi Jinping

In October 1950, barely a year after the Chinese civil war ended, Mao Zedong sent the first Chinese soldiers to fight in the Korean war. Between 180,000 and 400,000 of Chairman Mao's troops would die in that conflict, including his own son. But it was important to defend North Korea then, Mao reportedly said, because "without the lips, the teeth are cold".

- Amy Hawkins and Helen Davidson

That Chinese idiom has been used to described China and North Korea's close relationship for more than seven decades. China sees North Korea as a strategic security buffer in the region, while North Korea relies on its superpower neighbour for economic, political and military support. But that relationship is now under strain thanks to another war that is drawing Communist-rooted countries into a common battle.

Last month, the US revealed that it had seen evidence of thousands of North Korean troops being sent to Russia for possible deployment in the war in Ukraine. It later said at least 10,000 had reached Kursk province, and Ukrainian officials said some North Korean troops had already come under fire.

North Korea's entry to the theatre draws Asia closer into the European conflict, and causes significant headaches for China, which wants to maintain stability in the region, while grappling with a spluttering economy at home.