SEOUL - For the first time since leader Kim Jong Un took power in 2011, North Koreans were asked to take loyalty oaths on his birthday, a South Korean research institute said, amid other steps the country is taking to solidify his rule.

The loyalty oaths, which Reuters could not independently verify, were administered on what is believed to have been Kim's 40th birthday on January 8, according to the South and North Development Institute (SAND), a Seoul-based organisation that released photos of the oath in an ornate folder on Friday.

North Korea has never officially confirmed Kim's birth date, and traditionally such oath ceremonies have been held on the anniversaries of the birthdays of his father and grandfather, the nuclear-armed country's previous rulers.

"Kim Jong Un's choice to host a loyalty oath ceremony on his 40th birthday, as he begins his 13th year in power, signals a shift towards political assertiveness, departing from his predecessors' approach," SAND said in an analysis.

SAND's president, Choi Kyong-hui, told Reuters North Korea could move to designate Kim's birthday as an official anniversary as soon as next year.

The Kim family dynasty has ruled the country since its founding after World War Two, strengthening their grip on power by building cults of personality around them.

For the first time this year, North Korea stopped referring to the April 15 birth anniversary of founding leader Kim Il Sung as the "Day of the Sun," according to a Western tour agency that has partners in Pyongyang, and analysts who study state media.

"We should view this as part of North Korea’s effort to further bolster Kim Jong Un's leadership propaganda campaign," Rachel Minyoung Lee of the Washington-based 38 North programme, said of the decision to drop "Day of the Sun."

She noted that while such efforts are not new, they happen in phases over the years, with North Korea visibly accelerating efforts to play up Kim’s leadership in certain years.

Kim has also been showing off his daughter at official visits to everything from factories to missile launches, in what analysts said is aimed at bolstering the family's claim to power.

Last month North Korea released a new song featuring North Koreans of different backgrounds ranging from children to troops and medical staff exuberantly belting out lines such as: "Let's sing, Kim Jong Un the great leader" and "Let's brag about Kim Jong Un, a friendly father".

  - Reuters