Image caption The location where the torso was found is a well-travelled tourist area

Canadian police have launched a murder investigation after a woman's headless torso was discovered near Niagara Falls.

Passers-by spotted the body part floating in the lower Niagara River, police said.

Police have said the gruesome find is not related to a similar case where a woman's limbs were found scattered throughout Toronto earlier this month.

It is the third, unrelated Canadian case of dismembered body parts in 2012.

Preliminary results from a post-mortem examination revealed the latest victim to be a middle-aged caucasian woman.

Police say her body could have been in the water for between four and 10 days. They are asking for help in identifying the victim.

Authorities in the US state of New York and the Canadian province of Ontario - on either side of the Niagara River - are expected to review their missing-person files.

Niagara Police spokesman Constable Derek Watson told Canadian broadcaster CBC that citizens had seen a torso floating in the river between the Maid of the Mist and the Rainbow Bridge, two tourist attractions at the falls.

He said the police would ask the public and officials from Ontario and New York to help identify the victim.

"Over the next few days, as we're able to determine more things, we'll present it to the public to hopefully assist in identifying her."

Constable Watson also told the Toronto Star that investigators were looking at the possibility of a link with a missing persons case, although no such link has been made so far.

Earlier this month, police recovered limbs and a head belonging to 41-year-old Guang Hua Liu. Her estranged boyfriend, a construction worker from China, has been charged with murder.

In May, Canadian porn actor Luka Rocco Magnotta was arrested on suspicion of killing a Chinese student with an ice pick, carving up his body and sending his remains to schools in Vancouver, as well as political party offices in Ottawa.