Seoul, South Korea (CNN) -- President Barack Obama warned North Korea Sunday that if it moves forward with a planned test-firing of a long-range missile, it will further deepen its isolation, damage relations with its neighbors and face additional sanctions that have already strangled the country.

"North Korea will achieve nothing by threats or provocations," Obama said during a news conference in Seoul, South Korea, on the eve of an international nuclear security summit.

"North Korea knows its obligation."

Obama's strong words follow last week's announcement by North Korea that it is planning to carry out a rocket-powered satellite launch in April.

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The issue is overshadowing message of international cooperation for the summit, which is bringing together top officials from 54 countries including China and Russia.

South Korean President Lee Myung-bak said during the news conference that he considers the launch a direct violation of a U.N. Security Council resolution that bans the testing of the technology being used in the rocket-powered satellite.

South Korea has said it considers the satellite launch an attempt to develop a nuclear-armed missile, while the United States has warned the move would jeopardize a food-aid agreement reached with Pyongyang in early March.

"Bad behavior will not be rewarded," Obama said.

North Korea says it has a right to a peaceful space program and has invited international space experts and journalists to witness the launch.

A spokesman from the Secretariat of the Committee for the Peaceful Reunification of Korea denounced the South for working to turn the summit "into a platform for (an) international smear campaign" against the North, according to North Korea's state-run Korean Central News Agency (KCNA).

Pyongyang announced this month it would carry out a "satellite launch" in mid-April to commemorate the centenary of the birth of Kim Il Sung, the country's founder.

Using ballistic missile technology, however, is in violation of U.N. Security Council Resolution 1874 and against a deal struck with the United States earlier this month that it would not carry out nuclear or missile tests in return for food aid.

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Pyongyang has said it will see any critical statement of its nuclear program as "a declaration of war."

Prior to the news conference, Obama made his first visit to the demilitarized zone that splits the Korean peninsula. Obama peered through binoculars into North Korea where flags flew at half-staff to mark the 100-day anniversary of the death of Kim Jong Il.

"It's like you are in a time warp. It's like you are looking across 50 years into a country that has missed 40 or 50 years of progress," Obama said during the news conference.

The president said the real consequence for North Korea, should it go through with the launch, is that the country's leaders will miss an opportunity "to take a different path than the one they have been taking."

"I hope that at some point the North Koreans make the decision that it is in their interests to figure out how to feed their people and improve their economy rather than have big parades where they show off weapons," Obama said.

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The purpose of Obama's visit to the demilitarized zone was to meet with some of the 28,500 U.S. troops stationed in South Korea.

"When you think about the transformation that has taken place in South Korea during my lifetime, it is directly attributable to this long line of soldiers, sailors, airmen, Marines, Coast Guardsmen who were willing to create the space and the opportunity for freedom and prosperity," Obama told troops at a base near the DMZ.

It was Obama's first trip to the demilitarized zone, though he has made two previous trips to South Korea as president. Secretary of State Hillary Clinton paid a visit to the area in 2010.

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.