SEOUL, South Korea  In a Veterans Day speech at a United States Army base in central Seoul, President Obama said Thursday that America remains committed to defending South Korea, and warned North Korea that it faces continued isolation unless it fulfills its commitments to give up nuclear weapons.

While Mr. Obama is in South Korea for a Group of 20 meeting on fixing the stricken global economy, relations with North Korea are also expected to be on the agenda, particularly in talks with the host nation. The United States and its allies are seeking ways to get North Korea to return to six-party talks aimed at convincing the North to drop its weapons program.

Mr. Obama will meet later Thursday with the South Korean president, Lee Myung-bak, whose country’s relations with the North have become even more tense since the sinking in March of a South Korean warship, the Cheonan, apparently by a North Korean torpedo.