SEOUL--North Korean leader Kim Jong Un turned 40 on Monday with no announced public celebrations at home, after he entered the new year with artillery barrages into the sea and vows to expand his nuclear arsenal.

Since taking power in late 2011, Kim, the third generation of his family to rule North Korea, is believed to have established an absolute leadership similar to his predecessors. But his birthday has yet to be officially celebrated, unlike his late father Kim Jong Il and grandfather Kim Il Sung. Their birthdays are two of the North’s biggest holidays and are marked with great fanfare, loyalty campaigns and sometimes massive military parades.

On Monday, North Korea’s state news agency published a lengthy article extolling Kim’s guidance of major construction projects in the past decade. It also reported Kim visited a chicken farm with his daughter the previous day. But it made no mention of his birthday.

Some observers speculate Kim may think he’s still relatively too young or needs bigger achievements to hold such lavish birthday festivities. Others say the lack of a public birthday bash may be related to his concerns about attention to his late Japan-born mother.

Kim’s headlong pursuit of a bigger nuclear arsenal has invited punishing U.S.-led sanctions, which together with border closures during the pandemic were believed to have badly hurt the North’s fragile economy. Kim has subsequently admitted policy failures as his vow that North Korea would “never have to tighten their belts again” remained unfulfilled.

“For Kim, it’s still probably politically burdensome to idolize himself as he’s still young and hasn’t accumulated much achievements,” said Hong Min, an analyst at Seoul’s Korea Institute for National Unification.

Kim Yeol Soo, an expert at South Korea’s Korea Institute for Military Affairs, said it will likely take some time for his birthday to become an official holiday because elderly members of the North’s ruling elite would still think he’s too young.

Birthdays are central to the mythology of Kim Il Sung and Kim Jong Il, who had ruled North Korea with a god-like status since the country’s founding in 1948.

Their birthdays — April 15 for Kim Il Sung, and Feb. 16 for Kim Jong Il — are typically celebrated with tributes to their giant statues, dance parties, fireworks and art performances. On some milestone birthdays, North Korea’s military holds huge parades with goose-stepping soldiers and powerful weapons capable of targeting the U.S. and South Korea.

Kim Il Sung’s birthday was designated as an official holiday in 1968 when he turned 56, according to a website run by South Korea’s Unification Ministry, which handles relations with North Korea. Kim Jong Il’s birthday reportedly became an official holiday in 1982, when he turned 40.

North Korea has never formally commented on Kim Jong Un’s birthday. The only time Kim has been honored in public on his birthday was in 2014, when former NBA star Dennis Rodman sang “Happy birthday” before an exhibition basketball game in Pyongyang. The Unification Ministry-run website states that Kim Jong Un was born on Jan. 8.

There are also views that Kim may be worried about bringing unwanted attentions to his mother, Ko Yong Hui, a Japan-born dancer who was known as his father’s third or fourth wife. Ko’s links to Japan, which had colonized the Korean Peninsula in the past, and the fact that she wasn’t Kim Jong Il’s first wife, are considered as disadvantageous for Kim’s dynastic rule.

“The fact his mother came from Japan is his biggest weak point that undermines his legitimacy of the Paektu bloodline,” Park Won Gon, a professor at Seoul’s Ewha Womans University, said, referring to the Kim family’s lineage named after the country’s most sacred mountain.

“When Kim Jong Un’s birthday becomes an official holiday, he won’t still publicize details about his birth,” he said.

Despite no known public birthday events, experts believe Kim Jong Un faces little political challenge and is expected to intensify his run of weapons tests ahead of the U.S. presidential election in November.

In a key ruling party meeting in late December, Kim vowed to enlarge his nuclear arsenal and launch additional spy satellites to cope with what he called unprecedented confrontation led by the U.S. In the past few days, he had his troops fire artillery shells near the disputed sea boundary with South Korea, raising tensions with his rival.