North Korea on Monday added an important title to the fast-growing credentials of its untested new ruler, referring to the youngest son of late leader Kim Jong-Il as head of a key ruling party body.

In a move that experts said shows the successor is on track to take full control of the secretive nation, the ruling party newspaper hailed Jong-Un as head of its Central Committee.

"Let's stake our lives to safeguard the party's Central Committee led by dear comrade Kim Jong-Un," Rodong Sinmun said.

Jong-Un, in his late 20s, has already been touted as "great successor" and "supreme commander" of the military since his father died on December 17 of a heart attack at the age of 69.

Usually the head of the Central Committee -- a top decision-making body -- is also its general secretary, a position previously held by Kim Jong-Il.

"Jong-Un has not officially taken over as general secretary, but he is expected to inherit it and other posts held by his father," Dongguk University professor Kim Yong-Hyun told AFP.

The late Kim also chaired the all-powerful National Defence Commission and headed the 1.19 million-strong military.

Officially, Jong-Un's current highest post is vice-chairman of the party's Central Military Commission.

The North's official media heaped more praise on Jong-Un, describing him as a "tender-hearted man" who sent hot, sweet drinks to mourners braving freezing conditions in the capital Pyongyang.

The isolated state is making final preparations for what is expected to be an elaborate funeral for Kim on Wednesday that will be closely watched for clues about the powers at the side of the new ruler.

South Korea's Yonhap news agency reported Monday that Kim Jong-Il's eldest son Jong-Nam had arrived in Beijing from Macau and "placed under Chinese protection", quoting what it called a source familiar with his activities.

Seoul's National Intelligence Service said it had no information on the report and there was no other confirmation.

It was not clear whether the eldest son would attend Wednesday's funeral in Pyongyang, Yonhap said.

Kim Jong-Nam, 40, has lived abroad -- mainly in the Chinese territory of Macau -- for years after apparently falling out of favour with his father for trying to enter Japan on a false passport in 2001.

Two unofficial South Korean delegations crossed over the heavily militarised border into the North on Monday.

Lee Hee-Ho, the 89-year-old widow of late president Kim Dae-Jung, and Hyundai Group chairwoman Hyun Jung-Eun travelled overland to Pyongyang where they were to meet officials during their two-day trip.

"I hope that our visit to the North will help improve South-North relations," Yonhap news agency quoted Lee as saying before she crossed over with her aides.

They will not stay for the funeral and it was unclear whether they would meet Jong-Un.

Kim Dae-Jung and Kim Jong-Il held the first-ever inter-Korea summit in 2000 and Hyundai pioneered cross-border business projects.

A South Korean left-wing activist also left Saturday for North Korea via Paris and Beijing to pay her respects, her colleagues said, despite Seoul's ban on visits other than the trip by the former first lady and Hyundai chief.

The North on Sunday lashed out at South Korea for its response to Kim's death, warning of "catastrophic consequences" for relations unless Seoul eases restrictions on condolence visits by South Koreans.

The South blames its neighbour for two deadly border incidents last year, but has taken a generally conciliatory stance since the shock announcement on November 19 that Kim had died of a heart attack two days earlier.

But the authorities, who by law must approve all contacts with Pyongyang, are allowing only two private delegations to visit the North to pay respects and are not sending an official team.

The North says the world is in mourning for its late leader.

Chinese traders in Dandong, just across China's border with the North, told Yonhap Monday that North Korean business partners were urging them to offer condolences.

They said Pyongyang has set up mourning venues at its embassy in Beijing, its consular offices in Shenyang, Dandong and Yanji, and other places in China with a large North Korean population.