Five members of Taiwan's air force were missing on Monday after a military helicopter plunged into high seas during a humanitarian mission, the defence ministry said.
The helicopter had six crewmen on board when it plummeted into deep waters but one was successfully rescued, the ministry added.
The S70-C air force helicopter plunged into waters around 70 nautical miles (129.64 kilometres) southeast of Lanyu, an island in the southeast Taitung county, at 9.25pm local time.
It happened as the helicopter was trying to transport a sick sailor from an Indonesian-registered vessel, it said.
Rescuers retrieved one crew member but the rest, including two pilots, remained missing late on Monday.