U.S. President Barack Obama and South Korean President Lee Myung-bak met in Seoul Sunday and presented a united front against any possible threat from North Korea.

At a joint news conference after their meeting, Mr. Obama said North Korea would achieve nothing by making threats and said the country would only deepen its isolation if it carries out an announced rocket launch next month.

Mr. Lee agreed, calling the planned long-range launch a provocative action that threatens global peace and is against UN resolutions and an agreement with the U.S.

The comments came on the eve of an international nuclear security summit that will likely be overshadowed by North Korea's announcement that it will launch a satellite into space using a long-range rocket as part of celebrations marking the 100th anniversary of the April 15 birth of its late founder and President Kim Il Sung.

Earlier in the day, Mr. Obama visited the demilitarized zone dividing the two Koreas. The president, standing behind thick, bulletproof glass peered across the border into North Korea with the help of powerful binoculars.

He also met with American troops guarding the DMZ and told them they are a long line of soldiers who have enabled South Korea to prosper. He told the soldiers that South Korean President Lee Myung-bak had once confided he was able to rise from poverty as a child, to a successful career, thanks in large part to America's military aid and support.

President Obama visited the DMZ hours after arriving in South Korea, flying into at a U.S. air base south of Seoul. He will attend this week's global summit on nuclear security while in South Korea. He also plans various bilateral meetings with world leaders on the sidelines of the summit.

The North's nuclear ambitions, along with those of Iran and other countries will be the focus of the leaders of more than 50 nations at the summit, which begins Monday.

The U.N., U.S., European Union, Russia and Japan have warned North Korea that its scheduled rocket launch is in violation of U.N. resolutions, and urged Pyongyang to abandon the plan. Even North Korea's ally China has expressed concern that such a launch would undermine stability in the region.

The United States has said the launch would cancel an agreement to send North Korea a large shipment of U.S. food aid in exchange for halting its nuclear and long-range missile programs.

North Korea announced Saturday it would hold an annual parliamentary session April 13 amid preparations for the rocket launch.