China expressed shock at the death of North Korean leader Kim Jong-Il on Monday, as analysts said Beijing would do all it could to shore up its close ally, fearing instability on its border.

The foreign ministry in Beijing said it hoped North Koreans would "remain united" after their leader's death, and pledged to help maintain "peace and stability of the Korean Peninsula and the region".

"We are shocked to learn that DPRK top leader comrade Kim Jong-Il passed away and we hereby express our deep condolences on his demise and send sincere regards to the DPRK people," said ministry spokesman Liu Weimin, using the official name for the Stalinist state.

North Korea's official media announced on Monday that Kim, who suffered a stroke in 2008 but had appeared to recover, had died of a heart attack at the age of 69 and that his youngest son, Kim Jong-Un, would succeed him.

But there are fears in Beijing that Jong-Un has not had enough time to cement control over the country's government and military, and analysts said his father's sudden death would be a source of concern to China's leaders.

Jong-Un, who is in his late 20s, was given senior ruling party posts and made a four-star general in September 2010, despite his lack of any military experience.

"I think China will be very concerned because they were very keen to see a smooth succession between Kim Jong-Il and Kim Jong-Un," said Stephanie Kleine-Ahlbrandt, a Beijing-based analyst for the International Crisis Group.

"The idea was Kim Jong-Il would be around for another couple of years and would be able to fully put into place the mechanisms necessary for a transition of power. They have got to make sure he (Jong-Un) doesn't stumble."

Beijing is likely to strengthen support for its impoverished neighbour as it seeks to avoid a potentially destabilising power struggle in North Korea at a time when many of the country's 24 million people are starving, analysts said.

China is also likely to come under international pressure to use its influence to dissuade the unpredictable nuclear-armed regime from carrying out more military provocations like those that rattled Asia last year.

"It is dangerous in a sense that if the succession arrangement does not work there could be chaos," Professor Joseph Cheng of Hong Kong City University told AFP.

Beijing "certainly wants to avoid any kind of meltdown in North Korea because that would be destabilising along the border."

Observers in South Korea of the hermit state however played down fears of an immediate power struggle or a military coup.

University of California professor Susan Shirk said Chinese authorities would "do the most they can to be helpful," saying Beijing might also increase desperately needed aid to North Korea.

UN agencies have said that some six million people in the country urgently need food but a $73 million appeal for the North has only been 34 percent funded this year.

More than 21,700 North Koreans have fled their hunger-stricken homeland since the 1950-1953 Korean War, the vast majority in recent years as the food shortage worsened.

They typically escape on foot to neighbouring China before travelling to a third country. Beijing is worried that if the North Korea regime were to collapse, China could be flooded with millions of refugees.

Chinese news websites devoted extensive coverage to Kim's death, including family snapshots and photographs of Kim meeting senior Chinese leaders.

A black and white banner was splashed across the top of news web portals announcing the leader's death in a show of respect.

China's state CCTV television network also showed footage of North Koreans sobbing on the relatively deserted streets of Pyongyang.

Kim Jong-Il visited China twice this year, most recently in August when he told Beijing he was ready to resume six-party nuclear disarmament talks without preconditions.

But analysts said his sudden death at the weekend had dashed hopes of negotiations resuming any time soon.

"It will be some time before the new leaders of the DPRK will come back to the six-party talks," said Liu Youfa, vice president of the China Institute of International Studies, a government think tank.

"The domestic issues such as economic reforms are more pressing at the moment."

For several months there have been diplomatic efforts to restart the talks -- involving the two Koreas, the US, China, Russia and Japan -- which the North quit in April 2009, a month before staging its second nuclear test.