People in Pyongyang mourn Kim after his death was announced on Monday. [Photo/Agencies]

Kim's death was announced by state TV in a "special broadcast" from Pyongyang on Monday.

State TV said that the people and the military "have pledged to uphold the leadership of comrade Kim Jong-un", and called him a "great successor".

"All party members, servicepersons and people should remain loyal to the guidance of respected Kim Jong-un and firmly protect and further cement the single-minded unity of the party, the army and the people," the KCNA said in a broadcast.

The ROK's Yonhap News Agency said Pyongyang test-fired a short-range missile on its eastern coast on Monday.

Yonhap said the launch came in the morning, before DPRK state media announced the death of Kim. The last reports of short-range missiles being fired were in June.

Members of staff at Hae Dang Hwa, a famous Korean restaurant in Beijing, were unaware of the news when a China Daily reporter sought their reaction. Upon being told the news waitresses broke down in tears and the restaurant closed.

Kim was a frequent visitor to China, visiting twice this year alone, most recently in August when he stopped off while returning from Russia.

The ROK put its military on high alert. Lee Myung-bak, ROK president, convened a national security council meeting on Monday morning.

Tokyo's government spokesman offered condolences on the passing of Kim, before adding: "The Japanese government hopes that this unexpected development would not bring any adverse impact on the peace and stability of the Korean Peninsula".

At the same time the Japanese government held an emergency security meeting.

US President Barack Obama called the ROK's Lee at midnight on the US east coast, vowing to closely monitor events in the DPRK and cooperate.

A US State Department official told AFP that the US national security apparatus buzzed into life late on Sunday night.