Five construction workers were killed and two remain missing after a floating pier they were building overturned Sunday, throwing them into the sea off far-flung Okinotorishima Island, Japan's southernmost territory, the land ministry said.

Sixteen workers building the pier on the uninhabited atoll about 1,700 km south of Tokyo and 1,000 km southeast of Okinawa were dumped into the sea at around 7:30 a.m. and nine were rescued, the Land, Infrastructure, Transport and Tourism Ministry said.

The Japan Coast Guard sent two aircraft and two vessels to search for the missing. Due to vast distance between the atoll and the main islands, however, it will probably take the coast guard several days to arrive.

Construction of the 30-meter-long pier was commissioned to a venture comprising Penta-Ocean Construction Co., Nippon Steel & Sumikin Engineering Co. and Toa Corp., according to the ministry's Kanto Regional Development Bureau.

Most of the coveted but distant atoll is underwater. Only two of the so-called coral islets — Higashikojima and Kitakojima — can be seen during high tide.

Japan has taken steps to build up the islets, such as by building concrete levies to prevent the sea from covering them up at high tide, because they allow Japan to claim that it has an exclusive economic zone of roughly 400,000-sq.-km.

Marine experts believe the seabed around Okinotorishima Island is rich in natural resources.