Police have dropped around 70 percent of all reported rapes in the Oslo metropolitan area, citing an alleged lack of evidence. In the suburban Asker og Bærum police district, investigations into 20 of 28 reported rapes were dropped.

Of 170 reported rapes in Oslo and surrounding Akershus, only four resulted in convictions during the last three years, according to a new study.

"Generally speaking, many of these cases are difficult to prove," police inspector Stine Karlsen told Norwegian Broadcasting (NRK) on Tuesday. "There are seldom any witnesses, or technical evidence.

"It becomes a case of her word against his, regarding what actually happened," Karlsen added.

Meanwhile, police now believe that an assault late Sunday night on an 18-year-old woman in west Oslo may have been an attempted rape. The case was initially characterized as an attempted robbery.

The woman was attacked by a masked man on Sørkedalsveien, not far from the Vestre Gravlund cemetery. The physical assault has been described as brutal, leaving the woman with serious head injuries and a skull fracture in the facial area.

Police are seeking any witnesses who might have seen the man as he fled, just after midnight, reportedly up Sørkedalsveien and then left onto Guldbergs veien towards Monolittveien.

He was wearing military fatigues and a hooded black jacket.