TOKYO >>  Threatening lawsuits and protests, opponents are gearing up to fight a decision by Okinawa’s governor that could pave the way for a new U.S. military base on the southern Japanese island.

U.S. Defense Secretary Chuck Hagel welcomed the decision, calling it “the most significant milestone” so far in a long-running battle to realign U.S. forces in Okinawa.

The new base is designed to reduce the impact of the heavy U.S. military presence in Okinawa by replacing another base in a more congested area, but opponents want the operations moved off Okinawa completely.

“What the governor has done is unforgivable,” Yuichi Higa, the head of the assembly in Nago city, said in a phone interview. Nago would house the new base.

“Residents who are opposed will surely resort to the use of force, such as blocking roads, to stop this from happening,” Higa said.

Hiroshi Ashitomi, head of a Nago group opposing the base, said his organization would file a lawsuit challenging the governor’s decision.

Gov. Hirokazu Nakaima on Friday approved the Japanese Defense Ministry’s application to reclaim land for the proposed American base on Okinawa’s coast. It would replace the U.S. Marine Corps Futenma base in Ginowan city.

But he later told a news conference that he would continue pressing to move the Futenma troops off Okinawa entirely, noting estimates that it would take 9 1/2 years to build the base.

“My thinking remains it would be fastest to relocate outside (Okinawa) prefecture to a place where there is already an airport,” he said.

The new base is part of a U.S.-Japan agreement that would also move 9,000 Marines off Okinawa, including transferring 5,000 to Guam.

Hagel said the effort to realign American troops in Okinawa was “absolutely critical to the United States’ ongoing rebalance” to the Asia-Pacific region.

“Moving forward with this plan will reduce our footprint in the most populated part of Okinawa … while sustaining U.S. military capabilities vital to the peace and security of the region,” the defense secretary said in a statement.

The debate over the future of Futenma dates to 1996, when the U.S and Japan signed an agreement to close the base and move its operations elsewhere in Okinawa. In 2006, the two countries agreed to relocate the base to a relatively unpopulated area called Henoko in Nago city. But after the Democratic Party of Japan took power in 2009, it raised the possibility that the base could be moved off of Okinawa. While it later agreed to the Henoko plan, the proposal energized a movement to move the base elsewhere.

About half of the 50,000 American troops in Japan are based in Okinawa, and many residents complain about base-related crime, noise and the risk of accidents. Some feel the island is bearing an unfair share of the burden of protecting Japan from attack.

A key factor could be the outcome of a mayoral election in Nago next month that pits an opponent of the Henoko plan against a supporter.

“The governor is taking a risk putting the prestige of his office behind the project,” said Jun Okumura, a political analyst and former national government official. “I still don’t see the project going forward without the consent of the Nago mayor, but I see that this improves the chances of success.”

The rise of China’s military is reinforcing in the minds of some Japanese the need to have a strong defense, though it’s unclear whether it’s enough to sway public opinion in Okinawa.

“The government of Japan is poised to take whatever measures are necessary to maintain a strong deterrent while reducing the burden on the people of Okinawa,” said the government’s top spokesman, Chief Cabinet Secretary Yoshihide Suga.

Suga expressed his gratitude to Nakaima for what he called a “bold step.”

The politically difficult decision came only after Japanese Prime Minister Shinzo Abe met with Nakaima in Tokyo on Wednesday and offered him a package that included pledges of increased financial assistance for Okinawa.

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Associated Press writers Yuriko Nagano and Emily Wang contributed to this report.