China and other communist nations have not yet been invited to North Korea's rare party congress in early May, a diplomatic source with knowledge of the matter said Monday, amid signs that Pyongyang may conduct its fifth nuclear test ahead of the May congress.

North Korea is set to hold the Workers' Party Congress early next month during which the North's leader Kim Jong-un is expected to trumpet his policy of simultaneously pursuing economic development and nuclear weapons.

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

"As far as I understand, North Korea has not sent invitations to China, Cuba, Laos or Russia for the May congress," the source said on the condition of anonymity.

"Given the current circumstances, I believe that the possibility is low for North Korea to invite China to the May congress," the source said, citing soured relations between Pyongyang and Beijing since North Korea's fourth nuclear test in January.

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.

On Monday, South Korean President Park Geun-hye said signs of North Korea preparing for a fifth nuclear test have been detected.

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