Obama in South Korea visit as tensions rise with N Korea
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President Barack Obama salutes upon arrival at Osan Air Base to attend the Nuclear Security Summit, in Osan, south of Seoul, South Korea, Sunday, March 25, 2012. (AP Photo/Lee Jin-man)
US President Barack Obama has begun a visit to South Korea, amid rising tensions in the region over a rocket launch planned by the North.
Mr Obama first visited US troops in the Demilitarised Zone separating the Koreas, and is due in Seoul for talks.
The US has voiced concern that the rocket launch due in April is a pretext for a missile test. Pyongyang says it wants to put a satellite into orbit.
On Monday, Mr Obama will attend a security summit in Seoul.
The conference will be attended by leaders from more than 50 nations.
Its main focus will be preventing criminal or militant groups from acquiring nuclear weapons, the BBC's Lucy Williamson in Seoul reports.
Pyongyang's nuclear programme is not officially on the agenda. But American officials have made it clear that President Obama will be discussing the programmes of both North Korea and Iran in bilateral meetings with the Chinese and Russian presidents, our correspondent says.
North Korea is not taking part in the summit.
'In bad faith'
Earlier this week, Japan said it was readying its anti-missile defences ahead of North Korea's launch, expected between 12 and 16 April.
Pyongyang says the rocket - which would mark the 100th birthday of its late Great Leader Kim Il-sung - would take a new southern trajectory instead of a previous route east over Japan.
US Assistant Secretary of State Kurt Campbell has said an area between Australia, Indonesia and the Philippines could be affected by the flight path.
North Korea has shown a growing mastery of ballistic technology during its three previous long-range tests.
However, experts say none has succeeded in reaching orbit, and debris has fallen to earth at various stages during the launches.
Last month North Korea offered a deal involving the freezing of its nuclear and long-rang missile programmes in return for US food aid.
BBC diplomatic correspondent Jonathan Marcus says it was seen as a signal that the new leadership in Pyongyang wanted to explore the diplomatic track.
But Washington has accused Pyongyang of acting in bad faith following the rocket launch announcement.