China appears to have not been invited to a rare congress of North Korea's ruling party, a diplomatic source said Monday, reinforcing speculation that Beijing is unlikely to send a delegation to Pyongyang.

The first party congress in 36 years, which will start on Friday, is expected to display the North Korean regime's unity toward leader Kim Jong-un.

"There have been no signs that the Chinese government is preparing to send its delegation to the North Korean party congress," the diplomatic source said on the condition of anonymity.

"There have been no indications that North Korea invited foreign delegations to the party congress," the source said. "China also appears to have not been invited." 

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

Last week, China's foreign ministry spokeswoman Hua Chunying sidestepped repeated questions about whether China will send a delegation.

Instead, Hua said the upcoming party congress "is an important event for the North Korean people and we wish for its success."

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

Political ties between North Korea and China remain soured over the North's defiant pursuit of nuclear and missile programs.

North Korean leader Kim, who took power following the death of his father Kim Jong-Il in December 2011, is widely expected to use the rare congress as a venue to solidify his reign and show credentials for his policy of simultaneously pursuing economic development and nuclear weapons. (Yonhap)