Continuing his tour of Asia, President Obama has arrived in South Korea -- warning the North not to conduct another nuclear test.
The US leader's trip comes amid reports of increased activity at a nuclear site in the North.
Seoul says the intelligence suggests Pyongyang may be planning a fourth test.
Obama began his visit at the National War Memorial, paying tribute to soldiers from the South who died in the Korean War of 1950-53.
Earlier, in Japan, Obama called on China to dissuade its neighbour from continuing its nuclear programme.
There are fears that North Korea may look to disrupt the US president's visit with a show of strength.
Pyongyang is persistently irritated by joint US-South Korean military co-operation.
But it is also thought that the likelihood of irking Beijing -- North Korea's only major economic prop -- may deter it from actually carrying out a nuclear test.
Obama's visit comes as South Korea is preoccupied with the Sewol ferry tragedy in which hundreds of children died.
