Kim Jong-il, right, took power after the death of his father, Kim Il-sung, in 1994 [AFP]

International speculation is mounting over the health of Kim Jong-il, the North Korean leader, who has not been seen in public for several weeks.

Known in North Korea as the "Dear Leader", the 66-year old, who is rumoured to suffer from a range of chronic conditions, leads a secretive life and has never publicly named a successor.

But with doubts growing about Kim's wellbeing, attention is focusing on who might take over.

Here we take a look at some of the possible candidates.

Having taken over the reins from his father, North Korea's founder Kim Il-sung, Kim Jong-il heads the world's only communist dynasty.

Attention has therefore focused on the likelihood of one of his three sons being next in line to power. (Kim has several daughters, but given the highly patriarchal nature of the government it seems unlikely that any of them would be considered.)

Eldest son Kim Jong-nam was once considered the most likely succesor [AFP]

Kim Jong-il was himself anointed to power from his position as the eldest son of Kim Il-sung, and Korean society traditionally views the eldest son as the natural heir.

But there have been suggestions that Jong-nam was born illegitimately, casting doubt on whether he is, in fact, the next in line.

Jong-nam is also thought to have fallen out of favour after he was exposed trying to enter Japan on a false passport in 2001, apparently hoping to visit Tokyo's Disneyland.

He is thought to have based himself in Macau in recent years, spending much of his time at the Chinese territory's gambling tables and possibly having a hand in running North Korea's shady financial interests there.

With Jong-nam out of the frame, attention has shifted to Kim's second son, Kim Jong-chol.

Educated at a private boarding school in Switzerland, little else is known about him. Even his age is unclear, although he is believed to be in his late 20s.

According to some reports, Jong-chol has been seen accompanying his father in recent years on official trips, and he was reported to have met Hu Jintao, the Chinese president, during a visit to Pyongyang in late 2005.

But other reports have painted a different picture, with Kim reportedly branding his middle son "effeminate" and unsuitable for leadership.

Other analysts have also suggested Kim Jong-il's third and youngest son, Kim Jong-un, as a contender, although his age in a society that places emphasis on seniority could prove a drawback.

Beyond Kim's immediate family, another possibility is the North Korean leader's brother-in-law, 62-year-old Jang Song-taek.

The husband of Kim's sister, he fell out of favour in a power struggle with party reformers some years ago, but has recently returned to the inner leadership circle.

Although he officially holds only a relatively low rank, some North Korea watchers see him as the second-most powerful man after Kim in the ruling party structure.

Kim Yong-nam often represents North Korea at overseas summits [Reuters]

A long-time stalwart of the ruling Korean Workers' Party, 80-year-old Kim Yong-nam has for years been far more of a public face for North Korea than Kim Jong-il himself.

On most occasions it is Kim Yong-nam who receives visiting foreign leaders and represents North Korea at state visits and summits.

It was Kim Yong-nam, for example, who travelled to Beijing recently for the opening ceremony of the Beijing Olympic games.

But despite his standing, he is not thought to have the necessary revolutionary credentials to become anything more than a figurehead leader. 

Under what is known as the "army first" policy, ensuring it gets the lion's share of the nation's resources, North Korea's military is by far the most powerful force in the country and the only institution that can challenge the government.

As a result there has been much speculation that the country's next leader might come from the ranks of the military.

Kim has packed the senior ranks of the military with loyalists [AFP]

One Kim loyalist touted as a possible successor is General O Kuk-ryol, ranked second behind Kim in the powerful Central Military Commission.

A South Korean intelligence report published in 2006 said O was "seen as a very reliable person who also knows South Korea well".

Another potential succession candidate is Vice-Marshal Jo Myong-rok, head of the political department of the Korean Peoples' Army and seen as the second in command militarily to Kim Jong-il.

He is also head of the vast Chungwoonsan industrial conglomerate.

Jo stood alongside Kim Yong-nam and other senior dignitaries during the 60th anniversary parade at which Kim Jong-il himself was conspicuously absent.

In 2000, he led the highest level North Korean delegation ever to visit the United States, acting as a special envoy for Kim Jong-il in talks at the White House with Billl Clinton, the then US president.

Jo is seen as a staunch Kim Jong-il loyalist and one of his most trusted commanders, but at 82 he is also believed to be in poor health.

Anointing him as the country's new leader is likely to lead only to another succession debate within a relatively short period of time. 

The 2007 summit between North and South Korea. Is this the real Kim Jong-il? [Reuters]

In a book published this month, Toshimitsu Shigemura claims that Kim died in 2003, with his role taken over by a succession of body doubles.

Rumours have long circulated that Kim, fearing assassination, had selected up to four lookalikes, with candidates undergoing extensive plastic surgery to create the perfect likeness.

Shigemura says he has based his findings on tests of voice patterns from recent Kim appearances, which he says do not match the voice of the man who came to power in 1994.

The claim has been rejected by North Korean residents' groups based in Japan.

But in a country as secretive as North Korea, theories and rumours that might be considered outlandish anywhere else can easily flourish.