North Korean dictator Kim Jong Un believed to have turned 40

Gilsup Kwak, head of One Korea Center, a website specialising in North Korean affairs, suggests a number of reasons for the silence - not least, his mother Ko Yong Hui, and the fact she was reportedly born in Japan. There is still widespread distrust of Japan - which colonised the region between 1910 and 1945 - in North Korean society, and the link could undermine Kim Jong Un's bloodline claims, meaning it has so far been swept under the carpet. Making a big deal of his birthday, therefore, may bring unwanted attention. "Kim Jong Un's Achilles' heel is the truth of his birth," Mr Kwak explained.