BEIJING (AFP) - Delegates from China were absent from North Korea's once-in-a-generation party congress, Chinese media said Friday (May 6), in a potential sign of fraying ties between Pyongyang and its most important ally.

Beijing is a key supporter of the hermit kingdom, providing an economic lifeline that has allowed it to ride out waves of international sanctions.

But China's representatives were not invited to the gathering of North Korea's top political leaders, according to the Global Times, a newspaper with close ties to the Chinese Communist Party.

"North Korea wants to maintain its independent stance," professor Da Zhigang of the Institute of Northeast Asian Studies told the newspaper.

"It can't decide who to invite because it involves the interests of many sides." Details about the secretive party congress have been scant, and it is not clear if any foreign delegates were invited to attend.

The relationship between the two nations, once said to be as close as lips and teeth, has become increasingly strained since the death of North Korea leader Kim Jong Il.

His son, Kim Jong Un, has yet to visit China and frequently thumbs his nose at his ally, despite Pyongyang's reliance on Chinese trade.

A recent string of nuclear and ballistic missile tests have tried Beijing's patience.

But it has been reluctant to take measures against North Korea, fearing that they could destabilise the regime, creating a refugee crisis and swinging the regional balance of power towards the United States.

A large Chinese delegation attended the last Workers' Party congress in 1980, headed by Li Xiannian, later China's official head of state.

In an editorial, the Global Times said that China's relationship with the country is complicated.

Beijing is "resolutely opposing Pyongyang's nuclear development", it said.

"Many forces are now cursing the country, but China will never be one of them."