US President Barack Obama was on Tuesday to meet with the leader of South Korea, who is seeking security guarantees as a stand-off escalates with nuclear-armed North Korea.
The summit comes a day after the latest show of defiance by North Korea, which said that some 100,000 people rallied to denounce a tightening of UN sanctions on the hardline communist state for testing a nuclear bomb.
South Korean President Lee Myung-Bak has indicated that he wants Obama, who has set a goal of abolishing nuclear weapons, to reiterate that South Korea is under the US security umbrella.
Secretary of Defense Robert Gates reassured Lee in a meeting on Monday the United States was committed to defend South Korea "through all necessary means, including the nuclear umbrella," Lee's office said in a statement.
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The United States has 28,500 troops stationed in South Korea and more than 40,000 more in nearby Japan, which has tense relations with Pyongyang.
Lee, a conservative businessman, took over last year and -- delighting many in Washington -- reversed a decade-long "sunshine policy" under which South Korea put few restrictions on aid to the impoverished North.
