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.
Kim Jong-il died on Saturday of a heart attack, state media said. State TV is showing images of people filing past his open coffin in Pyongyang.
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.
In a statement on Monday, Beijing called on North Koreans to unite under the leadership of Kim Jong-un.
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.
Not much is known about the younger Kim, who is thought to be in his late twenties. He was first introduced in public as Kim Jong-il's earmarked successor in September 2010.
Cuba has declared three days of mourning for the North Korean leader.