At least two people have died and three others are missing after a helicopter returning from a firefighting mission crashed into the Douro River in northern Portugal, authorities said.

The pilot was rescued by a tourist boat near the crash site and taken to hospital. In addition to the pilot, there was also a team of five specialists from the Protection and Emergency Aid Unit in the helicopter when it crashed in the Lamega area.

"Two bodies were found inside the plane and three people are missing," Rui Silva Lampreia, commander of the Douro Port Authority, told reporters. "We are doing underwater searches around the helicopter and along the shores."

The cause of the accident is still unknown. Prime Minister Luis Montenegro traveled to the scene of the accident, where dozens of emergency services, maritime police, divers and two air force helicopters are present.