BEIJING, Dec 20, 2011 (AFP) - China's President Hu Jintao visited the North Korean embassy in Beijing on Tuesday to offer his condolences on the death of Kim Jong-Il, the official Xinhua news agency said.

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 "unite as one and turn their sorrow into strength".

The Chinese government, fearful of instability on its border, 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.

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.

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.

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

Streets outside the North Korean embassy 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."

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.