Construction vessels float on Oura Bay at the site of a future U.S. Marine Corps airfield at Camp Schwab, Okinawa, April 16, 2025. (Brian McElhiney/Stars and Stripes)

A construction vessel spilled fuel oil Thursday morning at the site of a future U.S. Marine Corps airfield in northern Okinawa, marking the fourth such incident this year, according to Japan’s coast guard.

The spill occurred at 8:15 a.m. while the vessel was transferring oil from its fuel tank to a gravity tank, the coast guard said in a news release. Oil overflowed onto the ship’s deck, and about 28 fluid ounces leaked into the sea.

The vessel was operating less than a half-mile north-northwest of Cape Henoko in Nago city at the time, according to the release.

A crew member reported the incident at 8:25 a.m., and workers used absorbent mats and other materials to contain the spill, which was under control by about 2:30 p.m., the coast guard said.

No injuries or environmental damage were reported, and the cause remains under investigation, according to the release.

Thursday’s incident follows two oil spills in January and another in March from vessels working on the same project. Also in March, a separate construction vessel caught fire while towing materials after oil leaked from its engine.

The airfield is being built on reclaimed land in Oura Bay to replace Marine Corps Air Station Futenma in the densely populated city of Ginowan. The relocation was agreed to by the U.S. and Japanese governments in 1996, but construction has faced repeated legal challenges from the Okinawa prefectural government.

The construction zone is split into two zones: 279 acres north of Camp Schwab and 91 acres to the south, according to the prefectural government’s website.