U.S. President Barack Obama began his visit to South Korea with a quick trip to the demilitarized zone separating the two Koreas, which technically remain at war nearly 60 years after an armistice.
The president of the United States greeted troops in the DMZ. "You guys are at freedom's frontier," Obama told them.
Pyongyang announced previously that it plans to send a satellite into space in the middle of April to coincide with the 100th birth anniversary of North Korea's founder, Kim Il Sung. The United States is among nations that believe North Korea will use the launch to test its ballistic capabilities.
The reclusive state has tried and failed twice before to place a satellite into orbit.
At a joint news conference Sunday, South Korean President Lee Myung-bak and President Obama said they agreed that if North Korea goes ahead with the launch, it will violate U.N. sanctions on Pyongyang's use of ballistic missile technology.
"North Korea will achieve nothing by threats or by provocations. North Korea knows its obligations and it must take irreversible steps to meet those obligations," said Obama.
But Obama added he is not sure who is really in charge in Pyongyang.
The primary reason for the president's visit to South Korea is to attend the Nuclear Security Summit, which is dedicated to keeping radiological materials out of the hands of terrorists. The nuclear programs of North Korea and Iran are not on the official summit agenda. However, those two countries are certain to be discussed by the leaders of the United States, China and South Korea during one-on-one meetings on the sidelines.
President Obama told reporters he will ask China's president, Hu Jintao, to put more pressure on North Korea to get Pyongyang to make a fundamental shift in its behavior.