Story highlights Authorities believe the plane either crashed or made an emergency landing, one official says

Air traffic control spokesman: "We don't have much hope now"

A spokeswoman for Norway's military says the aircraft was participating in a military exercise

Rescue crews are searching for signs of the plane

Search crews were looking for a Norwegian military aircraft that went missing over Swedish mountains Thursday, officials said.

The Hercules airplane from the Norwegian Air Force was taking part in a military exercise when it lost radio contact with authorities, said Per Froberg, a spokesman for Sweden's air navigation services. There were five people on board.

"We are working on two theories, either a complete crash, or that they have made an emergency landing and are unable to alert anyone," said Jonas Sundin, head of the air rescue operations in Gothenburg, Sweden.

The plane lost radio contact with authorities at 2:43 p.m. (9:43 a.m. ET), about 80 km (50 miles) west of Kiruna, Sweden, Froberg said.

Rescuers had found no signs of the plane more than four hours after it went missing, Sundin said.

"It has been quite a long time now since we lost radio contact, so we don't have much hope now," Froberg told CNN.

The plane was en route to Kiruna to pick up equipment and personnel when authorities lost contact with it, said Marianne Oiahals, a spokeswoman for the Norwegian Armed Forces.

"We have put all our resources in place to search for the plane," Oiahals said.

The aircraft was participating in a military exercise known as "Cold Response," she said.

Earlier Thursday, bad weather in the area forced a Norwegian rescue helicopter to turn back.

Two Danish helicopters, two Swedish helicopters and one Norwegian surveillance airplane were conducting the search Thursday evening, Sundin said.