Across the border on Sunday, North Koreans were mourning the death of Kim.
Kim's son, who took over from his father following his death in December, and senior party officials visited the Kumsusan Palace of the Sun in Pyongyang to observe a moment of silence, said North Korea's state-run Korean Central News Agency (KCNA).
The palace serves as the burial site for Kim Il Sung, the founder of North Korea, and his son, Kim, who succeeded him as the country's ruler.
Obama said he had not been able to assess the new leader of North Korea, saying "It's not clear exactly who is calling the shots and what their long-term objectives are."
Seoul's nuclear summit will be the second after Obama hosted the first meeting in Washington in 2010. He initiated the biennial summit after presenting his vision of a nuclear-free world in Prague in April 2009.
The official agenda will deal with nuclear terrorism and how to secure the world's nuclear material.
Obama's visit comes virtually two years to the day after the sinking of the South Korean warship, the Cheonan, which left 46 sailors dead.
South Korea says a North Korean torpedo attack was to blame for the ship's sinking. The North has denied the accusation.
CNN's Paula Hancocks, Shruti Pant, Jethro Mullen and Bob Kovach contributed to this report.
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