Obama warns North Korea against missile test launch
US President Barack Obama has warned North Korea that it will "achieve nothing by threats or by provocations".
The warning comes as Pyongyang prepares to launch a long-range missile which it says will put a satellite in orbit.
Mr Obama was speaking after talks in Seoul with South Korean President Lee Myung-bak, ahead of global summit on nuclear security.
The two leaders said North Korea risked further sanctions and isolation if it did not cancel its launch plans.
Mr Obama said Washington and Seoul were "absolutely united" that "bad behaviour" by North Korea would not be rewarded.
"North Korea knows its obligations and it must take irreversible steps to meet those obligations," he said.
The launch will contravene an agreement Pyongyang reached last month which would have seen it receive food aid in exchange for a partial freeze on nuclear activities and an end to ballistics tests.
Mr Lee, who spoke alongside Mr Obama, said their countries had "agreed to respond sternly to any provocations and threats by the North and to continually enhance the firm South Korea-US defence readiness".
But he said the international community stood ready to help North Korea improve the lives of its citizens if it chose a path of peace.
Mr Obama also criticised China, saying its refusal to challenge North Korea on the nuclear issue was not working as a policy.
In response to questions from journalists, the two leaders said it was hard to make an assessment of North Korea's new leader, Kim Jong-un, who came to power following the death of his father, Kim Jong-il, in December.
Mr Obama said it was "not clear exactly who is calling the shots" in North Korea and what their long-term objectives were, while Mr Lee said the planned rocket launch was a "disappointment".
The BBC's Lucy Williamson in Seoul says there had been hopes that the US aid deal and a new, young leader were indications the crisis could be moving towards resolution, but that with the announcement of the missile test, those hopes have gone.
The launch is scheduled for 12-16 April, to coincide with the 100th anniversary of the birth of the country late Great Leader Kim Il-sung.
South Korean defence officials say the main body of the rocket has now been moved to the launch site in preparation.
Earlier on Sunday, Mr Obama visited some of the US personnel based at the the heavily fortified Demilitarised Zone (DMZ) which separates the two Koreas.
The US has some 28,000 troops stationed in South Korea under a post-Korean War security alliance.
Mr Obama told the soldiers they were defending "freedom's frontier" and thanked them for helping to "create the space and the opportunity for freedom and prosperity".
Mr Lee is hosting more than 50 countries for a two-day summit on nuclear security in Seoul starting on Monday.
The summit's main focus will be preventing criminal or militant groups from acquiring nuclear weapons - North Korea is not officially on the agenda but is expected to feature in talks on the sidelines.
Meanwhile, North Koreans have been marking the end of 100 days of official mourning for Kim Jong-il. Tens of thousands of people gathered in Pyongyang to pay tribute to the leader, who died of a heart attack in December.