U.S. President Barack Obama will voice confidence on Monday that the United States can further reduce its nuclear weapons stockpile while maintaining its strategic deterrent and international commitments, a White House official said.

Obama, spoke at a university in Seoul ahead of global nuclear security summit, plans to raise the issue of arms control with Russian President-elect Vladimir Putin when they meet in May, the official said. Moscow and Washington reached a new START treaty earlier in Obama's term.

He will reaffirm his commitment to reduce America's nuclear weapons and the role they play in our national security strategy, the official said of Obama's address.

Obama set expectations high in a 2009 speech in Prague when he declared it was time to seek a world without nuclear weapons. He acknowledged it was a long-term goal, but his high-flown oratory helped him win the Nobel Peace Prize.

With Republican opposition as strong as ever to the United States joining the global nuclear test-ban treaty, Obama for now has had to shelve his earlier promise to push for ratification.

Another arms accord with Moscow will be an even tougher sell to conservatives who say Obama has not moved fast enough to modernize the U.S. strategic arsenal, a pledge he made in return for Republican votes that helped ratify the START treaty.

He unveiled a revamped policy in 2010 renouncing development of new nuclear weapons and restricting use of those already in Washington's arsenal. He followed that up by signing a landmark arms reduction treaty with Russia last year.

Obama secured commitments from world leaders at the inaugural 2010 Nuclear Security Summit in Washington to help keep bomb-grade material out of terrorists' hands, and independent experts say most of the pledges are being met -- though many were modest in scope.

But momentum seems to have slowed on Obama's nuclear agenda and, with the November 6 presidential election looming, chances for major new advances look doubtful.

Underscoring a sense of caution, defence and national security officials have spent months debating a secret set of new options being prepared for Obama to help guide future arms-control talks. Ideas range from maintaining the status quo to reducing warheads by up to 80 percent, an official has said.

But the administration appears reluctant to push publicly on such a divisive issue as his re-election campaign gathers pace.

CAUTION AGAINST COMPLACENCY

While arms-control groups sees some progress on nuclear security due to Obama's efforts, they caution against complacency when more than 50 leaders meet on Monday.

Outside experts are mostly sceptical of the chances for meeting the Washington summit's headline pledge to safeguard all of the world's nuclear materials within four years and are now pushing for voluntary arrangements to be made enforceable.

The main topic of conversation between world leaders in Seoul will be the nuclear programmes of North Korea and Iran, even though neither is on the official agenda and neither has been invited.

On Sunday, Obama ratcheted up pressure on China to rein in the North over its nuclear aspirations and missile programmes, saying it shouldn't turn a blind eye to its ally's bad behaviour.

Obama will meet his Chinese counterpart, Hu Jintao, and will urge him to use his influence over Pyongyang to try to convince the North not to proceed with a controversial long-missile test next month.

Seoul and Washington say the launch will be a disguised test of a ballistic missile that violates Pyongyang's latest international commitments. North Korea says it merely wants to put a satellite into orbit.

Obama said such a launch would only lead to further isolation of the impoverished North, which much show its sincerity if on-again-off-again six-party aid-for-disarmament talks are to restart.

China is host to the six-party talks, which involve Japan and Russia as well as the two Koreas and the United States. The talks broke down three years ago.

(Writing by Jeremy Laurence; Editing by Nick Macfie)