A college student was abducted and held as a sex slave for four days following two cases in which other female students were killed, police said over the weekend.

A 22-year-old woman surnamed Jin was abducted on Aug 21 and held as a sex slave for four days in Ji'nan, Shandong province. She was rescued on Aug 25.

Xinhua News Agency said Jin had left Qingdao in Shandong province by rail for her home in Tai'an, also in Shandong. While changing trains, she hired an illegal motorcycle from Ji'nan Railway Station to Ji'nan West Railway Station for 30 yuan ($5) and was abducted by the motorcycle driver, surnamed Dai.

Dai claimed he did not know the route to Ji'nan West station. He drove the woman around the city and she missed her train. She became angry and asked him to take her back to Ji'nan Railway Station, police said.

However, the 52-year-old did not take her to the train station, but raped her in a motorized tricycle and locked her up in his rented room. He beat and raped her many times until she was rescued, officers said.

Local police officer Liu Xiaoyi said the woman's eyes were swollen and she had multiple injuries.

"She was tied up at night while Dai fell asleep and she was monitored even when she used the bathroom. (The woman) was bound, choked, beaten, threatened and raped," Liu told China Central Television.

The victim took Dai's cellphone and sent an SOS message to a friend on Aug 25, police said.

Liu was one of the first officers on the scene. When police broke open the door, they saw "a young woman in her 20s crouching on a couch with obvious injuries to her face", Liu told China Central Television, adding that the woman was too scared to identify the suspect.

The suspect was calm and told police that the woman was his girlfriend, Liu said.

The victim said Dai had injured and raped her. Police said he confessed to his crimes against the student during their investigations.

In the past two weeks, two other female college students have been killed while making trips.

Police said Chongqing student Gao Yu, who missed a licensed taxi and took an illegal cab, was killed by the driver of the illegal cab during a dispute over the fare on Aug 19.

Gao Qiuxi, a student from Jiangsu province, was reported missing on Aug 12, on her way home from school. Nanjing police said on Thursday that Gao had been robbed and killed on Aug 12.