According to translations by BBC and Bloomberg, KCNA said the workers at the factory "broke into thunderous cheers of hurrah" when Kim appeared, but the report didn't mention his recent lack of appearances.
He was accompanied by his sister Kim Yo-jong and several other officials.
Mr Kim "said with deep emotion" that his grandfather Kim Il-sung and father Kim Jong-il "would be greatly pleased if they heard the news that the modern phosphatic fertiliser factory has been built", KCNA added.
Speculation has arisen that exactly something like this might be going on given that the Korean Central News Agency and Rodong Sinmun, North Korea's external news wire and ruling party newspaper respectively, have reported on Kim sending letters to foreign leaders amid his prolonged absence.
First, on the backs of a single-sourced report from the Seoul-based DailyNK suggesting that Kim had undergone a cardiovascular procedure and was in recovery, rumors exploded about his potential health.
State-run media in North Korea have not provided any definitive proof that Kim is alive, beyond reports of letters sent in his name. But until Saturday, it had gone weeks without reporting any public appearances by its leader or responding to the speculation about his health, and its silence fueled the rumor mill.
Trump refused to comment Friday on Kim's reported re-emergence.
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Kim attended the event on Friday in Sunchon, near the capital Pyongyang, after almost three weeks of swirling rumours that the leader of the nuclear-armed nation was seriously ill or possibly dead. "We'll have something to say about it at the appropriate time".
The North Korean dictator is reported to have attended a completion ceremony at a factory in Sunchon, in the South Pyongan Province, amid rumours he had died after failed heart surgery.
On Sunday, meanwhile, Moon Chung In, the top foreign policy adviser to South Korean President Moon Jae In, told USA media that Kim is well and has been staying in Wonsan since April 13.
But others question whether core members of North Korea's elite, mostly men in their 60s or 70s, would find it hard to accept a young and untested female leader who lacks military credentials.
Ji Seong-ho told Yonhap News Agency he believes that North Korea will announce Jong Un's death this weekend. But most have turned out to be groundless.
South Korean Unification Minister Kim Yeon-chul, who oversees engagement with Pyongyang, said it was plausible Kim was absent as a precaution over the coronavirus pandemic, in view of the stringent steps taken to head off an outbreak in the country.