Five workers died and two others were missing in an accident at a construction site on Okinotorishima, Japan's southernmost island, officials at the Japan Coast Guard and transport ministry said.
The accident occurred around 7:30 a.m. Sunday when a metal pier tens of meters long overturned while being unloaded from a barge, the ministry said.
Of 16 workers at the site, nine survived, though some were injured, ministry officials said.
The government set up a liaison office in the crisis control centre at the Prime Minister's Office.
Winds were not particularly strong at the accident site at the time, while estimated wave height was about two meters, according to the Japan Meteorological Agency.
Okinotorishima, located about 1,700 kilometers south of Tokyo, has a coral reef of 4.5 kilometers east-west and 1.7 kilometers north-south. It is surrounded by a 400,000-square-kilometer exclusive economic zone.
Japan started construction work in fiscal 1987 to prevent the island from eroding and being submerged by the sea.
Work to install the pier for ocean research ships and other vessels has been under way since Aug. 21 last year and is scheduled to be completed Sept. 30.