The United States plans to transfer about 4,000 of 19,000 marines stationed in Okinawa Prefecture to Guam over a period from 2024 to 2028, the head of the U.S. Pacific Command said Wednesday.

Adm. Harry Harris told a congressional hearing that “the movement of the bulk of the marines to Guam would occur in the 2024 to 2028 time frame.”

Japan and the U.S. agreed in 2013 to start moving marines from Okinawa to Guam in the first half of the 2020s as part of efforts to reduce its footprint on the prefecture, which hosts the bulk of U.S. military bases in Japan.

But the transfer is effectively tied to the relocation plan for Marine Corps Air Station Futenma within Okinawa, a contentious project that is supposed to lead to the base’s closure and the return of its land to Japan. The plan is fiercely opposed by residents who want the base moved outside the prefecture.

Speaking at the House Committee on Armed Services, Harris said the U.S. also plans to transfer about 3,000 marines to Hawaii from Okinawa.

Harris expressed hope that the U.S. military will reduce the number of marines in Okinawa “ultimately” to “a point around 10,000 or 11,000.”

“The whole issue of moving marines from Okinawa elsewhere is important to our alliance relationship with Japan,” he said.

“Japan has invested a lot in this,” he said. “This is all about — for everyone else’s benefit — reducing the footprint in Okinawa, and also closing Futenma.”

Harris said last year that the U.S. expects a two-year delay to 2025 in the plan to move the Futenma base from crowded Ginowan to a less populated coastal area of Nago, given the strong local opposition.

Speaking at Wednesday’s hearing, Harris quoted Japanese officials as saying a replacement facility will be ready by Japan’s 2022 fiscal year, but said, “I testified last year that I thought that that was in question.”

Okinawa Gov. Takeshi Onaga has demanded that the Futenma base be moved off the prefecture instead of to a replacement facility, the construction of which will lead to the partial landfill of waters off Henoko.