Hwang, 64, was formerly a top official in the ruling party's Organisation Guidance Department with a portfolio that included the physical and political protection of Kim.

The leadership change comes amid growing concerns that North Korea is preparing to conduct a fourth nuclear test.

"To all intents and purposes, this makes Hwang, who also has close personal ties to Kim Jong-un, the second most powerful man in the country," said Michael Madden, author and editor of the NK Leadership Watch website.

Hwang's appointment came just days after KCNA reported his promotion to the rank of vice-marshal on April 28 - a rank shared with Choe and just four others.

According to Madden, Hwang's name started to emerge in the mid-2000s around the time Kim's father, Kim Jong-il, was finalising arrangements for who would succeed him as leader.

"He became one of Kim Jong-un's mentors and has been close to him for around 10 years, so he's been groomed for this role," Madden said.

It was not immediately clear what had become of Choe, who was widely believed to have moved into the role of North Korea's unofficial No2 following the execution in December of Kim's uncle and political mentor, Jang Song-thaek.

Choe holds a number of other top positions as a Politburo Standing Committee member and a vice-chairman of both the Central Military Commission and the National Defence Commission.

There were multiple reports earlier this year that Choe had been arrested and possibly purged after he dropped from public view for three weeks - an unusually long absence for such a senior figure.

His reappearance in March, alongside Kim, led to speculation that his disappearance might have been due to health problems.

"I don't think we can say Choe has been purged, but he's clearly been moved aside, possibly for health or other reasons," said Yang Moo-jin, a professor at the University of North Korean Studies in Seoul.

Yang agreed that Hwang was now effectively No2 in the country, given his military rank, position and personal access to the supreme leader.

Coinciding with news of the apparent reshuffle yesterday was a report by a US think tank that North Korea had been testing engines for intercontinental ballistic missiles.

The US-Korea Institute at Johns Hopkins University said satellite images of the North's Sohae rocket launch site suggested one "and maybe more" recent tests on the engine of what is probably the first stage of a road-mobile ICBM called the KN-08, a missile with a targeted range of up to 11,000 kilometres. That range puts the entire United States within range.

Stop violating human rights, UN diplomats tell North Korea

North Korea must act immediately to halt a litany of abuses and crimes against humanity, diplomats said during a UN review of the nation's rights record.

But North Korea - backed by its main ally, China - hit back at the criticism yesterday and said a recent report by UN investigators was designed to defame the country.

Diplomats at the UN Human Rights Council in Geneva voiced outrage at the "systematic, widespread and gross rights violations" detailed in the report, which documented grave abuses in the country, including extrajudicial killings, torture and sexual violence.

"We note with concern that … human rights violations and crimes against humanity continue to take place with impunity," British representative Karen Pierce said.

North Korea has categorically rejected the report, with one of the country's top diplomats Choe Myong-nam insisting yesterday that it was "full of distortions".

Several countries praised North Korea's advances in reducing child mortality and allowing reunions between families separated by conflict on the Korean peninsula.

But US ambassador Robert King told the council that Washington was "alarmed by the widespread use of forced labour, including child labour, in detention facilities".

He called on Pyongyang to "acknowledge the human rights violations in the country and take immediate steps to end such violations".

Counsellor Chuandong Chen, of China, urged the international community to treat North Korea "in a fair manner".

North Korea's representative to the UN in Geneva said progress had been made in a number of fields since its last review, which all 193 UN countries must undergo every four years.

He pointed to new legislation to protect the rights of children, women and the disabled, a broadening of the state education and health-care systems, expanded efforts to provide enough food to the often famine-struck nation and a free housing programme.

But, he said, efforts to protect and promote human rights were threatened by "the persistent politically motivated pressure and military threat by outside forces", and "aggravating economic sanctions" imposed by the international community.