The U.S. military is in the midst of an $11 billion project to modernize its aging bases in South Korea and move most of its troops farther from the heavily fortified border with North Korea as it solidifies plans to keep a strong presence here for decades to come.

U.S. officials have described the bases being built in South Korea as the biggest military construction effort since the digging of the Panama Canal. Some 100 installations scattered across the country are being consolidated into 50 locations, aligned with two major hubs.

Most of the 28,500 U.S. troops stationed in the country will move to about 40 miles south of the Han River and Seoul, the capital, to Camp Humphreys. The move will leave South Korean troops in the vanguard of allied defenses close to the demilitarized zone, the no-man’s land that has bisected the Korean Peninsula since the Korean War ended in a truce in 1953.

The most visible change will be in Seoul, where the U.S. military will vacate its headquarters inside the Yongsan Garrison, in the heart of the city, handing over a huge chunk of prized real estate to South Korean officials. U.S. forces have occupied the site for six decades. As the city has grown up around it, Yongsan’s low brick walls topped by rusty concertina wire have looked increasingly out of place amid the bustle of the modern Asian metropolis.

Personnel from the military headquarters are among those moving to Camp Humphreys, which is expected to house 44,000 people — including troops, civilian employees and family members — making it the largest U.S. garrison in Asia.

The base realignment has been delayed repeatedly. It was originally supposed to be finished by 2008, but the completion date has been pushed back to 2016, and some projects could be postponed beyond that, according to U.S. defense officials.

South Korea is covering most of the estimated $11 billion in construction and relocation costs, in recognition of the extensive security benefits provided to the country by the U.S. military. The South Korean and U.S. governments have been reluctant to release detailed figures, but an April report by the Senate Armed Services Committee found that the Pentagon’s share of the costs could total $3.2 billion.

The expenses are a tall order for both governments, but they pose a particular challenge to the Pentagon, which is grappling with $1 trillion in forced budget cuts over the next decade.

Nonetheless, Defense Secretary Chuck Hagel said several times during a visit to South Korea this week that the United States is not considering any reduction in the number of its forces in the country. He cited the security threat posed by North Korea, which conducted a nuclear test in February and is developing long-range ballistic missiles thought to be capable of reaching the western United States.

“Our focus on our interests and on our commitments to our allies will remain the same,” Hagel told reporters here Wednesday.

At the direction of President Obama, the Pentagon has been trying to devote more strategic attention to Asia by bolstering its alliances and military presence in the region. Hagel said the Asia strategy remains a priority, but he acknowledged that budget pressures are forcing the Pentagon to make hard choices elsewhere.

“We’ll adjust,” Hagel said Wednesday at a news conference at the Ministry of Defense in Seoul. “You always adjust your resources to match your priorities, and we will continue to do that.”

The cost of maintaining U.S. troops in South Korea has been rising steadily for years, apart from the construction bills related to the base realignment. Last year, the Pentagon spent $2 billion in military personnel costs and $1.1 billion in other expenses to support its forces in the country, according to the Senate report.

South Korea provided an additional $765 million in funding last year under a burden-sharing agreement with the United States. But Seoul’s share has been rising at a much slower rate than Washington’s in recent years, the Senate report noted.

That agreement is up for renegotiation this year. Kim Kwan-jin, the South Korean defense minister, declined to say whether his government would be willing to shoulder more of the burden.

“The exact amounts and levels of the costs have yet to be negotiated,” he told reporters.