China is unlikely to send a delegation to North Korea's rare ruling party congress early next month, diplomatic sources said Wednesday, as the North's nuclear ambition and missile launches have soured political ties between the allies. 

North Korea announced earlier in the day that the Workers' Party Congress will start on May 6, the first such congress in 36 years. 

North Korean leader Kim Jong-un is expected to use the rare congress as a venue to solidify his reign and take credentials for his policy of simultaneously pursuing economic development and nuclear weapons. 

China, North Korea's diplomatic and economic lifeline, sent a vice premier to North Korea's party congress in 1980.

"So far, it has been understood that no invitation was sent to China," a diplomatic source said on the condition of anonymity. 

North Korea has been slapped with tougher international sanctions since early last month over the January nuclear test and launch of a long-range rocket in February.

Many analysts suggest that North Korea could conduct another nuclear test to bolster domestic solidarity in time for the May party congress. 

Another diplomatic source said North Korea's apparent preparations for a fifth nuclear test might be a burden for China. 

"If North Korea goes ahead with the fifth nuclear test, it would be difficult for China to oppose demands by the international community, including South Korea and the U.S., to take stronger actions," the second source said.

In recent months, North Korea has claimed progress in its defiant pursuit of nuclear weapons and ballistic missiles, including what it said was the successful launch of a ballistic missile from a submarine. (Yonhap)