"Chief Leader Kim Jong Un cuts ties for the completion of Sunchon phosphate fertilizer plant," the Korean Central Utility Agency (KCNA) reported.
KCNA says it was a "completion ceremony of the factory" on Friday, "the international holiday for the working people around the world."
KCNA released a still image that allegedly shows Kim at the ceremony. The picture shows him cutting a red ribbon, with sister Kim Yo-jong standing behind him. CNN cannot independently confirm the authenticity of the photograph or the date it was taken.
North Korean state newspaper Rodong Sinmun said on Sunday that Kim sent thanks to workers who helped rebuild the city of Samjiyon.
CNN cannot independently confirm the authenticity of Sunday's report and whether the thank-you note came directly from the leader.
The same newspaper said Friday that Kim had "thanked" exemplary party members for their efforts to promote party politics. The article did not provide details on when or how his feelings were expressed.
It is not the first time North Korean media reports on Kim's activities since news of his potential health problems emerged.
US President Donald Trump declined to comment on the appearance report on May Day.
"I'd rather not comment on it yet, Kim Jong Un. We want something to say about it at the right time," he told reporters before leaving Camp White for the White House.
On Sunday, South Korean presidential adviser Moon Chung-in told CNN that the North Korean leader was "alive and well" despite speculation about his well-being.
The adviser to South Korean President Moon Jae-in said Kim had been staying in the Wonsan area on the country's east coast since April 13, adding: "No suspicious movements have been detected so far."
A website specializing in North Korean affairs published satellite images Saturday as researchers said a train, "probably belonging to Kim Jong Un," had been parked at a train station serving Kim's luxurious Wonsan connection on the country's east coast since April 21.
CNN's Betsy Klein and Jake Kwon contributed to this report.