The U.S. military continues to have a robust presence in South Korea, in part to serve as a

The U.S. military continues to have a robust presence in South Korea, in part to serve as a deterrent to the North. Obama on Friday declared the alliance between the U.S. and South Korea “a linchpin of security in Asia.”

“Our solidarity is bolstered by the courage of our service members, both Korean and American, who safeguard this nation,” Obama said during a news conference Friday with South Korean President Park Geun-hye.

Ahead of his meetings with Park, Obama paid tribute to U.S. soldiers killed in the Korean War. He placed a wreath beside a plaque bearing the names of some of those killed as a bugler played out taps.

Obama will speak Saturday at Yongsan Garrison, headquarters for U.S. forces in South Korea. Before his remarks, Obama and Park will have a rare joint leaders briefing with the commander of the U.S-South Korea Combined Forces Command.

Both countries are closely watching North Korea, which has threatened to conduct its fourth nuclear test. Obama and Park both warned Friday that the launch could lead to tougher sanctions, with Park also declaring that it could trigger an undesirable nuclear arms race in the region and render further nuclear negotiations pointless.

The website 38 North, which closely monitors North Korea, said commercial satellite imagery from Wednesday showed increased movement of vehicles and materials near what are believed to be the entrances to two completed tunnels at Punggye-ri nuclear test site. The movements could be preparations for an underground atomic explosion, although predicting underground tests is notoriously difficult.

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Associated Press writers Darlene Superville in Seoul and Matthew Pennington in Washington contributed to this report.

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