Hundreds of protesters stage a rally opposing the approval on the U.S. base relocation by Okinawa Gov. Hirokazu Nakaima on Dec. 27. (AP/AP)

For nearly two decades, fierce local opposition has stalled a plan to relocate a controversial U.S. Marine base on the island of Okinawa. But on Friday, Japan’s hawkish prime minister persuaded Okinawa’s governor to sign off on the construction of a replacement facility — a step that was hailed in Washington as a diplomatic breakthrough.

The concession by Gov. Hirokazu Nakaima is a significant achievement for Prime Minister Shinzo Abe, whose government spent months lobbying for the move. To win Nakaima’s support, Abe offered new spending for infrastructure and development projects on Okinawa and pledged to help reduce the island’s troop-hosting burden.

The struggle to find a replacement site for Marine Corps Air Station Futenma had become a source of friction between Japan and the United States, and in recent years some Obama administration officials had grown pessimistic about resolving the impasse. Abe’s ability to do so highlights the power he has consolidated one year into his term, a period in which he has riled neighbors and worked to beef up Japan’s defense.

Nakaima signed off Friday morning on plans for landfill work that will allow the Futenma air station to be moved to a less populated area. Tokyo and Washington agreed on the relocation site seven years ago.

But in accepting the deal, Nakaima warned that the relocation will not be easy and said he would still prefer the base to be moved off the island entirely. Some analysts say the relocation could be held up by court challenges. They also caution that aircraft crashes or crimes by U.S. service members could change the political atmosphere, making it harder for Nakaima — or his successors — to implement the deal.

Following the approval, roughly 2,000 protesters flocked to the Okinawan prefectural assembly building in the city of Naha, local media reported, holding signs saying, “Leave office, governor” and “We won’t allow the landfill.”

Still, U.S. officials greeted the agreement as a major step. The relocation of Futenma is a key element of a broader U.S. realignment of troops and resources in the Asia-Pacific region, where the administration is seeking to augment the U.S. presence in the region to counterbalance China’s military rise and anticipate threats from a volatile North Korea.

“Reaching this milestone is a clear demonstration to the region that the alliance is capable of handling complex, difficult problems in order to deal effectively with 21st century security challenges,” Secretary of Defense Chuck Hagel said in a statement, referring to the relationship the United States and Japan forged after War World II.

Completion of the new air station, which is expected to include two mile-plus-long runways, will take approximately a decade, U.S. officials said. As the new facility is built, the Pentagon expects to reduce the number of Marines based in Okinawa from 18,000 to roughly 10,000, as more are deployed to Guam, a U.S. territory, Australia and Hawaii.

U.S. officials have pressed Japanese leaders for years to clear the way for the construction of a new base along Okinawa’s rural northern coast. The Futenma base currently sits in the middle of densely populated Ginowan city, surrounded by schools and homes. Residents say it poses safety concerns, citing a 2004 incident in which a U.S. helicopter clipped a university administration building. Some also complain about noise pollution and the sometimes unruly behavior of U.S. troops. Washington and Tokyo have been trying to move the base for nearly two decades.

Most Okinawa residents are opposed to opening a new U.S. military facility on an island that already hosts 34 of them. Okinawa makes up less than 1 percent of Japan’s total land area but hosts half of the 50,000 U.S. troops stationed in the country. In 2009, then-Prime Minister Yukio Hatoyama pledged to move Futenma off Okinawa, citing the prefecture’s disproportionate burden. His move failed, but in the process it hardened opposition to the internal relocation.

Until Hatoyama’s pledge, Nakaima, the governor since 2006, supported the relocation plan. But he faced increasing pressure to stop it, and for several years said publicly he would never allow it. However, some officials in Tokyo believed that Nakaima was a pragmatist willing to negotiate.

“He was just looking for the right bargain, and that’s what he got,” said Patrick Cronin, senior director of the Asia-Pacific Security Program at the Center for a New American Security.

Londoño reported from Washington. Yuki Oda in Tokyo contributed to this report.