BEIJING (AFP) - A top North Korean official made an unexpected visit to China on Tuesday (May 31) in an apparent attempt by Pyongyang to mend frayed ties with its powerful neighbour, Japanese media reported.

Beijing is North Korea's largest trading partner and has been its key diplomatic protector for decades, but ties have been strained recently by Pyongyang's internationally-condemned nuclear test program, with Beijing supporting UN sanctions against its isolated neighbour.

Ri Su Yong, vice chairman of the North's ruling Workers' Party, arrived in Beijing to brief Chinese officials on a once-in-a-generation party congress held earlier this month, Kyodo news reported.

The lack of any official Chinese representation at the congress - which cemented leader Kim Jong Un's grip on power - was viewed as a sign of friction between the two traditional allies.

The visit came even as South Korea said Tuesday the North failed in what was believed to be an attempted launch of a powerful new medium-range missile.

UN resolutions ban North Korea from any use of ballistic missile technology, although it regularly fires short-range missiles into the sea off its east coast.

China's official Xinhua news agency said Ri - a former foreign minister - would visit for three days as part of a delegation.

Ri is the highest-ranking North Korean official to visit China since last year when Kim Jong Un's close aide Choe Ryong Hae attended a military parade in Beijing to mark 70th anniversary of Japan's defeat in World War II, Kyodo reported.