North Korean state media outlets remained silent on the presumed 40th birthday of their leader Kim Jong-un on Monday.

The Korean Central News Agency, Rodong Sinmun, Korean Central Television and Korean Central Broadcasting Station have yet to make any report on Kim's birthday.

Kim is known to have been born on Jan. 8, 1984, though the North has not officially confirmed the date.

Earlier in the day, state media outlets carried reports on Kim's visit to a newly built chicken factory where he called for increasing poultry production for a "substantial change" in the people's standards of living.

Kim was accompanied by his daughter, known as Ju-ae, which marked her rare public appearance at an economy-related visit. She had mostly accompanied her father to military events.

The North's foreign ministry highlighted Kim's diplomatic achievements in a statement on a summit between Kim and Chinese President Xi Jinping that took place on Kim's 35th birthday in 2019.

The statement, which assessed the visit as helping strengthen their bilateral ties, however, did not mention Kim's birthday.

South Korea's unification ministry in charge of inter-Korean affairs said it will keep tabs on any notable movement.

"North Korea does not have a tendency of highlighting or mentioning Kim Jong-un's birthday. While many presumptions can be made on such intention, we will not jump to conclusions and continue to monitor," Koo Byoung-sam, the ministry's spokesperson, told reporters in a press briefing.

North Korea celebrates the birthdays of Kim's late grandfather and father, Kim Il-sung and Kim Jong-il, as national holidays.

Kim Jong-il saw his birthday designated as a national holiday in 1982, when he was 40, eight years after he was designated as the successor to his father and national founder Kim Il-sung.