Mourners in North Korea have been paying their last respects at the open coffin of late leader Kim Jong-il. State TV showed pictures of his son and heir, Kim Jong-un, and other senior officials at a memorial palace. Kim Jong-il died on Saturday of a heart attack caused by overwork and stress at the age of 69, state media said. A period of official mourning is under way. Kim Jong-il, who had been in power since the death of father Kim Il-sung in 1994, will be buried on 28 December. North Korean sate media said its people continued to grieve late into the night. How much of this is genuine rather than highly choreographed propaganda is difficult to know, says the BBC's John Sudworth in Seoul. Kim Jong-il had been in the process of formalising his third son, Kim Jong-un, as his successor. But the transition had not been completed and analysts fear Mr Kim's death could trigger a period of instability in the internationally isolated nuclear-armed state. The worry is that Kim Jong-un, less than 30 years old and a complete unknown quantity, may lack the political skill to hang on to power in this nuclear-armed state; a power struggle with other family members or the military elite is a worrying possibility, our correspondent says. Kim Jong-un was first introduced in public as Kim Jong-il's earmarked successor in September 2010. Regional worries The US has called on North Korea to pursue a "path of peace" following the death of leader Kim Jong-il. Secretary of State Hillary Clinton said the US was ready to help the North Korean people and create lasting security on the Korean peninsula. Around the region, countries have been holding urgent talks about what the implications of Mr Kim's death might be. South Korea, whose military has been put on alert in the wake of Mr Kim's death, has sent its condolences to the North Korean people, but says no official delegation will be travelling from Seoul to Pyongyang to pay their respects. The government has also increased its alert level against possible cyber attacks from Pyongyang. But it says it will reconsider a plan to display Christmas lights near the border - something that angers North Korea. Chinese President Hu Jintao, meanwhile, has visited the North Korean embassy in Beijing to offer his condolences, China's Xinhua news agency said. Late on Monday, the Chinese foreign minister also met the North Korean envoy in Beijing, the agency reported. China is North Korea's closest ally and its biggest trading partner. It is keen to avoid a power struggle that could bring further instability to the region or any kind of collapse that would send a tide of North Korean refugees across its border. Kim Jong-un, hailed by North Korean media as the "Great Successor", is believed to have visited China on at least one occasion in recent months. Cuba has also declared three days of mourning for the North Korean leader.