WASHINGTON – Washington has told Tokyo that the Okinawa Prefectural Government should unconditionally let landfill work begin in Nago so Futenma air base can finally be moved, diplomatic sources said.

The message, delivered by a senior U.S. official, puts more pressure on Okinawa Gov. Hirokazu Nakaima, who is to decide this month whether to approve the central government’s application to fill in land off the coast of the Henoko district so a new airstrip can be built.

Washington is worried the lengthy stalemate over moving U.S. Marine Corps Air Station Futenma out of densely populated Ginowan will drag on if Nakaima attaches conditions to the project, the sources said Sunday.

Conditional approval could slow the realignment of U.S. forces in Japan, given congressional pressure to cut defense spending, a source in the U.S. government said.

The senior official also relayed plans to the prefecture to give more support for Okinawa, such as reducing the heavy concentration of U.S. forces there, if Nakaima gives unconditional approval, the sources said.

While the governor’s decision is impossible to predict, observers say he could request that the Status of Forces Agreement be reviewed or seek the tentative deployment of Futenma’s marines elsewhere in Japan as a condition for approval.

But Nakaima, who wants the Futenma base out of the prefecture, is in a bind because the U.S. has vowed to keep the base where it is unless he approves the plan, and residents fiercely oppose keeping it in Okinawa.

Since taking office for the second time in December 2012, Prime Minister Shinzo Abe has stepped up pressure on Okinawa to accept the unpopular relocation plan.