At least four people died and one was missing after a helicopter returning from a firefighting mission crashed into the River Douro in northern Portugal on Friday, authorities said, Paralel.Az reports citing Reuters.

The pilot was rescued alive by a tourism boat near the scene and was taken to a hospital with leg fractures.

Aside from the pilot, the helicopter was carrying a team of five specialists from the Emergency Protection and Relief Unit when it crashed in the Lamego area.

"Two bodies were inside the aircraft and another two were found near the tail of the aircraft, which was broken in two," Rui Silva Lampreia, commander of the Douro Harbour Authority, told reporters.

"We're doing underwater searches around the helicopter and on the banks," he said.

The cause of the accident was yet to be established.

Prime Minister Luis Montenegro, who travelled to the scene of the crash, told reporters Saturday would be a national day of mourning.