Chinese President Xi Jinping has dispatched a senior official to Pyongyang to reassert China's "deep friendship" with North Korea, as concern rises in Beijing and Washington over Kim Jong Un's blossoming alliance with Russian leader Vladimir Putin.

The visit by Zhao Leji, the third-ranking member of the Chinese Communist party's leadership group, the politburo standing committee, marks the highest-level meeting between China and North Korea in nearly five years.