Archeologists said on Monday they have found what they believe are traces of a Viking ship buried in southeast Norway, a rare discovery that could shed light on the skilled navigators' expeditions in the Middle Ages.

The boatlike shape was detected about 50 centimetres underground in a tumulus, a burial mound, with the use of a ground-penetrating radar in Halden, a municipality located southeast of Oslo.

"In the middle of the mound, we discovered what is called an anomaly, something that is different from the rest and clearly has the shapes and dimensions of a Viking ship," Knut Paasche, an archeologist at the Norwegian Institute for Cultural Heritage Research (NIKU), told AFP.

"What we cannot say for sure is the condition of the conservation. Yes there was a boat there, but it's hard to say how much wood is left," Paasche said.

The Vikings, Northern European warriors and merchants who sailed the seas between the 8th and 11th century, would bury their kings and chiefs aboard a boat hoisted onshore and left under a mound of earth.

Only three Viking ships in good condition have been discovered in Norway in the past, including the well-preserved Oseberg ship discovered in 1903. All three of them are now exhibited in a museum near Oslo.

"We need more discoveries to explain what these boats looked like and how the Vikings would sail," Paasche said.

Left without a bow and a stern, which is the back part of a ship, the traces were discovered in Halden prior to agricultural drainage operations. They are about 20 metres long, potentially making it one of the largest Viking ships discovered in Norway, according to NIKU.

The institute says it now considering what to do with the discovery, but ruled out an excavation at this time of the year.