US President Barack Obama and South Korea’s President Lee Myung-bak during a joint news conference at the Blue House in Seoul on Sunday. Image Credit: Reuters

Seoul : Warning North Korea from its doorstep, US President Barack Obama said Pyongyang risks deepening its isolation in the international community if it proceeds with a planned long-range rocket launch.

"North Korea will achieve nothing by threats or provocations," Obama said during a news conference yesterday in South Korea's capital of Seoul, where he was to attend a nuclear security summit.

Obama spoke following a private meeting with South Korean President Lee Myung-bak. Both leaders warned there would be consequences if North Korea proceeds with next month's launch, with Obama saying the move would jeopardise a deal for the US to resume stalled food aid to the North.

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

Earlier yesterday, Obama paid his first visit to the tense zone separating North and South Korea amid new nuclear tensions. He told American troops stationed nearby they are protectors of "freedom's frontier".

Obama shook hands and spoke briefly in the dining hall at a US military camp just outside the 4km-wide Demilitarised Zone, then walked into the heavily patrolled no-man's land to tour a small post where South Korean forces patrol just 100 metres from the demarcation line.

Obama, positioned behind bulletproof glass, peered through binoculars across the line that has bisected the Korean peninsula for 60 years. He spent about 10 minutes at the observation post, looking first towards North Korea, then back to the South.

It was an unmistakable show of force to communist North Korea and its new leader at a time of diplomatic standoff. Obama underscored the Cold War symbolism by making the tour his first order of business ahead of a gathering of world leaders pledged to keep nuclear materials safe. Nuclear-armed North Korea will not attend.

The US is threatening to cancel planned food aid to the North over its announcement that it will launch a long-range rocket next month, news that overshadows the meeting of world leaders committed to nuclear security that Obama will attend in Seoul.

"I could not be prouder of what you're doing," Obama told smiling American troops at Camp Bonifas at the edge of the DMZ.