US President Barack Obama said Sunday it was unclear who was "calling the shots" in North Korea under its new young leader and stepped up demands for Pyongyang to abort its planned rocket launch.

Obama stood with South Korean President Lee Myung-Bak to present a united front against the communist North, hours after staring into what he termed a "time warp" as he visited the last land border left over from the Cold War.

The US leader also had some unusually public criticism of China for its failure to induce its North Korean ally to open its nuclear programme to inspections and end years of "provocations" and "bad behaviour".

"It is hard to have an impression of Kim Jong-Un, in part because the situation in North Korea still appears unsettled," Obama said of the man proclaimed "great successor" after the death of his father Kim Jong-Il last December.

"It is not clear exactly who is calling the shots and what their long-term objectives are," he told a press conference, in Washington's frankest assessment yet of Pyongyang's murky power politics.

His comments deepened speculation about the elevation of Jong-Un, aged in his late 20s, and raised the alarming prospect of a power struggle in a volatile and erratic nation armed with nuclear weapons.

What was clear, Obama said, was that the North's leaders "have not yet made that strategic pivot where they say to themselves, 'What we are doing isn't working. It is leading our country and our people down a dead end'."

The president earlier got an up-close look into the isolated Stalinist state when he climbed a clifftop observation post 25 metres from the demarcation line that has divided the Koreas for six decades.

After squinting through high-powered binoculars from behind a bulletproof screen over a border guarded by mines, barbed wire and tank traps, Obama said he had stared into a "time warp".

He then turned towards a huge North Korean flag flapping in the stiff breeze at half-mast to mark the 100th day since Kim Jong-Il's death, and to a horizon dotted with rudimentary buildings peeking through the haze.

The visit, during which Obama told some of the 28,500 US troops guarding South Korea that they stood at "freedom's frontier", was meant as a firm show of unity with Seoul and appeared partly aimed at Kim Jong-Un.

Obama, in South Korea for a 53-nation nuclear security summit, joined Lee to stiffen a call for North Korea to halt a satellite launch next month to mark the 100th anniversary of the birth of founding leader Kim Il-Sung.

"North Korea will achieve nothing by threats or by provocations," Obama said, adding that Pyongyang would deepen its isolation by firing off a rocket.

Washington says this would really be a missile test that would flout UN resolutions and scupper a recently agreed US food aid deal.

Lee was equally blunt.

"President Obama and I have agreed to respond sternly to any provocations and threats by the North and to continually enhance the firm South Korea-US defence readiness," he said.

Obama also delivered an unusually blunt critique of China's unsuccessful attempts to rein in its volatile neighbour, which has defied the world for years by developing nuclear weapons despite a punishing battery of sanctions.

His comment came a day before he is due to meet Chinese President Hu Jintao for talks in Seoul, and appeared to be another sign of Obama's increasing frustration with Beijing as he sets his sights on a second White House term.

Though he said he sympathised with China's desire for stability on its border, Obama argued that efforts to "paper over" North Korea's repeated belligerence and defiance had not worked.

"My suggestion to China is that how they communicate their concerns to North Korea should probably reflect the fact that the approach they have taken over the last several decades has not led to a fundamental shift in North Korea's behaviour," Obama said.

He also delivered a scathing assessment of life in North Korea, saying it was decades behind its southern neighbour and would remain so until it made a "strategic" pivot and accepted offers of help in return for ending its nuclear programme.

Obama said the North was guilty of a long cycle of provocations designed to win concessions from the West.

"President Lee and I have agreed from the start of our relationship that we are going to break that pattern," Obama said.