* Hagel tours zone under watchful eye of northern troops
* 'No margin for error up here'
* North Korea seen watching U.N. response on Syrian chemical arms
PANMUNJOM, Korean Demilitarized Zone, Sept 30 (Reuters) - U .S. Defense Secretary Chuck Hagel toured the Korean DMZ on Monday, at times under the watchful eye of North Korean soldiers, and said the Pentagon had no plan to reduce its 28,500-member force in the South despite budget constraints.
"This is probably the only place in the world where we have always a risk of confrontation," Hagel said after touring a single-story building with a corrugated metal roof where talks are held with North Koreans on Conference Row in the truce village of Panmunjom.
As Hagel walked through the building, which spans the military demarcation line between North and South, two North Korean soldiers peered through the windows on the northern side filming his movements.
"There's no margin of error up here," Hagel told reporters after walking through the structure. "It's a very important location that we need to pay attention to."
Hagel also visited the hilltop Observation Post Ouellette in the demilitarized zone (DMZ), where he looked across a valley into North Korea and received a briefing from South Korean Defense Minister Kim Kwan-jin. The post is named for Private Joseph Ouellette, who won the Medal of Honor in the Korean War.
The U.S. defense secretary's visit to the demilitarized zone came on the first full day of a four-day trip to South Korea to celebrate the 60th anniversary of a mutual security alliance between the two countries.
Hagel will participate in talks about the future of the alliance with his South Korean counterpart and will attend a change-of-command ceremony for U.S. forces in South Korea. He will be joined Army General Martin Dempsey, the chairman of the Joint Chiefs of Staff, and Admiral Samuel Locklear, the head of U.S. Pacific Command.
Army General Curtis Scaparrotti, the former director of the U.S. Joint Staff, will take over as commander of U.S. forces in Korea from Army General James Thurman.
Hagel told reporters that while the Pentagon is under pressure to reduce projected spending by nearly a trillion dollars over the next decade, the U.S. military had no plan to reduce the size of U.S. forces in Korea.
"No, there's never been any consideration of changing our force protection or force presence here in Korea or anywhere else in this area," Hagel said, noting U.S. President Barack Obama's desire to refocus on the Asia-Pacific after a dozen years of war in Iraq and Afghanistan.
"We'll continue to do what we've got to do to manage those (spending) reductions, (and) at the same time assure our partners ... specifically here in the Asia-Pacific that our commitments still stand," Hagel said.
The U.S. defense chief said he thought North Korea, which is believed to have large stockpiles of chemical weapons, had been watching developments surrounding Syria's use of chemical weapons in its civil war. But he said it was difficult to know what lessons Pyongyang might draw.
The United Nations adopted a resolution last week demanding that Syria eliminate its chemical weapons. The vote came amid outrage over a sarin gas attack in a Damascus suburb that killed hundreds. Facing the threat of U.S. military action and coaxing from Moscow, Syria agreed to surrender the arms.
"I think it's pretty clear that North Korea has been carefully observing the activities, especially of last week at the United Nations," Hagel said. "Nations who possess those kinds of weapons and who are irresponsible do watch how the world responds and reacts."
Before visiting the DMZ on Monday, the U.S. defense chief watched an exercise in which U.S. and South Korean troops used live ammunition and explosives to destroy an obstacle so their tanks and armored vehicles could advance.