Obama leaves on trip to Asia
Air Force One carrying the president and his aides took off from Andews Air Force Base outside Washington at 00:07 am local time (0407 GMT).
Obama wants to pay tribute to some of the 28,500 US soldiers serving in South Korea.
He also wants to honor the strength of their host nation, a key ally in Asia, a region to which he has reoriented American foreign policy.
The president will on Sunday meet South Korean President Lee Myung-Bak ahead of a 53-nation nuclear security summit in Seoul and a day before holding more key talks with China’s President Hu Jintao, officials said.
On Monday he will hold his last meeting as an equal with outgoing Russian President Dmitry Medvedev with whom he masterminded a reset of US ties with the Kremlin.
He will hold talks too with the leaders of Turkey and Kazakhstan.