Kim's absence could mean the state has yet to meet some of its policy objectives in the run-up to the congress, South Korean news service Newsis reported.
Kim's last public appearance according to state media was on April 23 or 24, when he was a "witness" to the "successful" launch of a submarine-launched ballistic missile, or SLBM.
Analysts in the South say the silence could mean the North has been unable to yield more concrete results in weapons tests and construction ahead of the congress.
Seoul had told press last week a mid-range North Korean missile, the Musudan, had failed during launch. The test and its results were not made public in North Korea media.
The congress is to mark a turning point in Kim Jong Un's leadership. The North Korean ruler has emphasized the dual pursuit of economic and nuclear weapons development.
But China, a close ally, has not signaled any intention to send an envoy to the meeting, which is taking place for the first time in more than three decades.
Yonhap reported that Beijing could have either declined an invitation or Pyongyang did not extend a request to attend.
Relations between the two countries have deteriorated since China supported the passage of sweeping economic sanctions at the United Nations Security Council in March.
At the last party congress in 1980, North Korea drew 177 delegates from 118 countries.
Despite its increased diplomatic isolation, North Korea has not stopped issuing verbal threats of a nuclear attack against South Korea and the United States.
On Monday, a North Korean foreign ministry spokesman told state outlet DPRK Today that, "the choice is now up to the United States, to respond to our demands, or accept a baptism of nuclear fire."
North Korea has previously called for a peace treaty with the United States but Washington has made denuclearization a higher priority.