TAIPEI -- A coastguard was killed Friday afternoon after a rescue helicopter, with five on board, lost control and plunged into waters off New Taipei City in northern Taiwan, authorities said.

The coastguard was declared dead at hospital. The pilot was under emergency treatment, while the remaining three are in a stable condition, the local fire department said.

On Friday afternoon, the helicopter was exploring an oil spill from a vessel that became stranded at about 9 am Thursday.

A video clip showed the helicopter hovering some 20 meters above the water, but it suddenly starts to spin before dropping into the sea.

Coast and fire teams discovered all five on board -- the pilot, a co-pilot, two coast guards and a crew chief.

Owned by a Taipei-based company, the stranded vessel from north Taiwan's Keelung was 10 years old and was carrying 617 containers and 447 tonnes of fuel.

All 21 of the ship's crew had been saved by 1 pm Thursday.