North Korea has sent a top military official to China as a special envoy of leader Kim Jong-Un, at a time of strained relations between Pyongyang and its allies in Beijing, state media reported.

Choe Ryong-Hae, the director of the Korean People's Army politburo flew to China on Wednesday, the Korean Central News Agency said, without detailing the agenda or length of his visit.

Choe is believed to be the highest ranking North Korean official to visit China since late leader Kim Jong-Il in August 2011. Kim Jong-Un has never visited China since he took over after his father's death in December 2011.

The trip comes at a sensitive time for North Korea-China relations which have been sorely tested in recent months by Pyongyang's refusal to heed Beijing's warnings over its nuclear weapons programme.

China has long been the impoverished, isolated North's sole major ally and chief economic benefactor, but Beijing sided with the rest of UN Security Council in imposing sanctions after the North's long-range rocket test in December last year, and its nuclear test in February.

The sanctions triggered an escalating cycle of military tensions on the Korean peninsula, during which China came under enormous US-led pressure to rein in its wayward ally.

In line with the UN sanctions, Beijing has moved to tighten Pyongyang's financial operations in China which the international community points to as the major conduit for funding its nuclear weapons programme.