NEW YORK — President Barack Obama warned North Korea Sunday about its planned rocket launch next month and called on China to help curb Pyongyang’s nuclear ambitions, according to media accounts.

Speaking in Seoul, South Korea, Obama said North Korea would be breaking its global obligations if it conducts the long-range rocket launch, and warned the move would force Washington to ditch a recent agreement to grant North Korea much-needed food aid in exchange for initial talks over its nuclear program.

“North Korea will achieve nothing by threats or provocations,” Obama told a news conference in the South Korean capital, Reuters reported.

In Seoul for a nuclear security summit organized by South Korea, the meeting that begins Monday was convened to consider ways to prevent terrorists from getting nuclear weapons and fuel, but would likely be dominated by concerns about North Korea and Iran’s nuclear plans.

Obama planned to meet Russian counterpart Dmitry Medvedev and China’s Hu Jintao during his three-day trip to Seoul, with the president on Sunday saying China would “have to act” to help prevent North Korea from developing nuclear arms, Bloomberg reported.

“I believe that China is very sincere that it does not want to see North Korea with a nuclear weapon. But it is going to have to act on that interest in a sustained way,” said Obama, according to Reuters.

Sunday marked the 100th day since of the death of North Korean leader Kim Jong Il, succeeded by his son, Kim Jong-un.

Obama spoke after making his first visit to the demilitarized zone dividing North Korea and South Korea, where he used binoculars to peer into North Korea, then visited U.S. troops stationed nearby.

Speaking at the news conference with South Korean President Lee Myung Bak, Obama likened his glimpse into North Korea to seeing a “time warp” of a half-century of missed opportunity, according to Bloomberg.