President Lee Myung-Bak's visit to Washington was planned well ahead of the North Korea crisis and aimed in part at smoothing out relations after Obama in his campaign rejected a hard-negotiated free-trade deal with Seoul.

The global economic crisis, which has hit South Korea hard, is also expected to be on the agenda between the two nations whose alliance has dramatically broadened in recent years to include working together on Iraq and Afghanistan.

Mr Obama 'looks forward to exploring ways in which the two countries can strengthen cooperation on the regional and global challenges of the 21st century,' said Robert Gibbs, the White House press secretary.

But North Korea is set to take center-stage after the hardline communist state tested a nuclear bomb and long-range missile and stormed out of a six-nation disarmament agreement.

The UN Security Council voted unanimously Friday to toughen sanctions on North Korea, leading some experts and officials to fear Pyongyang will carry out another nuclear test in retaliation.

Mr Lee, who starts his talks on Monday meeting Secretary of State Hillary Clinton, has enraged the impoverished North by refusing aid without progress in the nuclear and other rows - reversing a decade of liberal policies by Seoul.

'I don't think you'll see any daylight between President Lee and President Obama in terms of North Korea,' said Victor Cha, who was former president George W. Bush's top aide on Pyongyang.

'As a former negotiator, I have to tell you that is music to our ears because that wasn't always the case,' said Mr Cha, now at Georgetown University and the Centre for Strategic and International Studies (CSIS) think-tank.

Quoting official sources in Seoul, Yonhap news agency said Obama - who advocates abolishing nuclear weapons - would publicly reaffirm the US nuclear umbrella in Asia amid jitters in both South Korea and fellow US ally Japan. South Korea gave up work on nuclear weapons in the 1970s under US pressure. -- AFP