TOKYO >> Okinawa’s Gov. Denny Tamaki renewed demands Thursday that Japan’s central government halt construction of a U.S. Marine Corps. base being relocated to a less-crowded area of the southern Japanese island despite vehement local opposition.

Tamaki was responding to a defense ministry estimate that the project will require more than twice the time and costs earlier estimated because the seabed at the planned reclamation is “as soft as mayonnaise,” experts say, and needs reinforcing.

U.S. Marine Corps Air Station Futenma is to be moved from densely populated Ginowan to the previously undeveloped Henoko area on Okinawa’s eastern coast. Futenma’s current base is to be closed and returned to Okinawa. Opponents of the relocation plan want the base moved entirely out of Okinawa

“In order to achieve a closure and return of Futenma air station as soon as possible, the construction work like this should immediately stop,” Tamaki told reporters.

Delaying the relocation of the base adds to safety risks for the crowded Ginowan area, one of the main reasons for moving Futenma.

The Defense Ministry said moving Futenma base to Henoko will cost $8.5 billion and take 12 years, pushing its completion and the closure of Futenma into the 2030s. That adds more than a decade to the plan, which has already been delayed by more than 20 years because of local opposition and other reasons.

Under an earlier plan agreed to by Tokyo and Washington in 2013, construction was to cost about $3.2 billion and take five years, with completion expected in about 2022.

Chief Cabinet Secretary Yoshihide Suga on Thursday defended the relocation plan as “the only solution, taking into consideration the role of the Futenma air station as deterrence under the Japan-U.S. alliance and a removal of its risks.”

Most of the additional cost and time is required to stabilize and strengthen reclaimed land off the coast of Henoko that will be used for runways, the Defense Ministry said. It presented its new estimate Wednesday to a panel of Japanese experts.

The heavy U.S. military presence on Okinawa is a longstanding source of conflict between the island and Washington and Tokyo.

About half of the 50,000 American troops in Japan are stationed in Okinawa. The 30 U.S. installations on the island account for more than 70% of the area used by the U.S. military in Japan, leading Okinawa to protest that it is shouldering more than its share of the burden.

Japan’s central government forcibly began reclamation work in December 2018 despite repeated protests by Okinawans.

Tamaki says Tokyo’s approach is high-handed and undemocratic. He has called for a three-way dialogue between Okinawa, Tokyo and Washington. He is expected to reject an application by the central government for a local government permit to carry out additional land reinforcements. That would likely reignite tensions and further delay the relocation.

Opponents of the relocation plan also contend it should be scrapped for environmental reasons because the site is a habitat for certain corals and for dugongs, a marine mammal similar to a manatee that the International Union for Conservation of Nature says is critically endangered, just a step away from extinction.

Washington and its ally Tokyo reaffirmed their commitment to pursue the Henoko plan in 2017, saying it was the only way to end the use of Futenma. The plan was developed after the 1995 rape of a schoolgirl for which three U.S. servicemen were convicted, reigniting simmering Okinawan opposition to the U.S. bases.

Tokyo and Washington have agreed the current 1,100-acre (445-hectare) Futenma base will be returned to Japanese control after operations are moved to Henoko. The plan requires 5,900-foot runways built in a V configuration on reclaimed land in Henoko Bay near the U.S. military’s Camp Schwab.