China's president offered his condolences for the death of Kim Jong-Il Tuesday as Beijing, fearing instability on its border, called on North Koreans to unite behind their former leader's son and heir.

Hu Jintao visited the North Korean embassy in Beijing in person to pay his respects, a day after the isolated Stalinist state announced the death of its leader from a heart attack.

China is North Korea's closest ally, and just hours after the announcement of Kim's death on Monday, Beijing threw its backing behind his third son Jong-Un, urging North Koreans to "turn their sorrow into strength".

The Chinese government also pledged to work with the isolated, nuclear-armed state to ensure the "peace and stability of the Korean Peninsula and the region".

"We believe that under the leadership of the Korean Workers Party and comrade Kim Jong-Un, the DPRK people will unite as one and turn their sorrow into strength," said Foreign Minister Yang Jiechi, using communist North Korea's formal name.

Yang has held talks on the phone with US Secretary of State Hillary Clinton and South Korean Foreign Minister Kim Sung-hwan on the importance of ensuring security on the Korean Peninsula after the reclusive leader's death, foreign ministry spokesman Liu Weimin said Tuesday.

Analysts say China is likely to strengthen support for its impoverished neighbour as it seeks to avoid a potentially destabilising power struggle in the Stalinist regime.

Beijing is also likely to come under international pressure to use its influence to dissuade Pyongyang from carrying out more military provocations like those that rattled Asia last year.

Little is known about Kim's successor, but there are fears he has not had enough time to cement control over the country's government and military.

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.

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

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

Kim was a regular visitor to China and his death was splashed across the front pages of China's state-run newspapers on Tuesday. It was also the most popular topic on the weibos -- the Chinese version of Twitter.

"Beijing and Pyongyang have enjoyed a long tradition of friendship, forged by generations of leaders," said an editorial in the China Daily.

"It is hoped our neighbour will continue its endeavour to build an economically stronger DPRK under a new leadership."

The China Daily carried a photo of a smiling Kim under a headline reading "A friend's departure".

The People's Daily, the mouthpiece of China's Communist party, carried news of Kim's death, but no commentary.

Liu told reporters that Beijing would "welcome the leaders" of North Korea "to visit at their convenience", underlining China's support for Kim Jong-Un.

Streets outside the North Korean embassy in the Chinese capital were blocked to cars and bicycles on Tuesday morning as Hu visited.

The only pedestrians allowed inside the cordoned area were North Koreans clutching bunches of white chrysanthemums -- the traditional flower for mourning -- wrapped in clear plastic.

Police told an AFP reporter at the scene that the embassy would remain off limits to non-North Koreans for at least one day.

"You can't have a visa today," a policeman told AFP. "Their leader has died."

North Korean restaurants were closed but the Golden Garden Flower Shop across the street from the embassy -- one of the few florists in the area -- was doing a roaring trade as dozens of North Koreans flocked to the store to buy bunches of fresh flowers and wreaths.

The floor was covered with debris and piles of flowers were stacked on tables in the cramped store as three people worked non-stop to make flower arrangements for the mourners.

Sombre-looking North Koreans waiting to buy flowers refused to speak to AFP.

A florist said that business had been "very good" since Kim's death.

"Can't you see? It's busy," the woman told AFP as she frantically cut flower stems.