A man charged over the deaths of two Toronto gay men is a suspected serial killer, police said, as they linked him to three additional victims and scoured sites around Canada's largest city for more remains.

Landscaper Bruce McArthur, 66, was arrested earlier this month and charged yesterday with three additional premeditated murders.

Human remains were found "hidden in the bottom of (large) planters" at a property that he used for storage, Toronto police's lead investigator Sergeant Hank Idsinga said.

Mr McArthur is an "alleged serial killer," he told a news conference, adding that authorities were searching 30 properties in Toronto where he worked.

Police are examining a dozen more planters from various locations across the city, and examining two properties for excavation "where people might be buried".

"We believe there are more remains at some of the properties that we're working to recover," Mr Idsinga said.

"We believe there are more (victims) but I have no idea how many more there are going to be."

DNA tests are being performed on the dismembered skeletal remains found so far "and depending on the identification of the remains and further evidence that we uncover, we'll lay more charges," he said.

Mr McArthur was arrested following an investigation into the disappearances last April and June of two men from a predominantly gay neighbourhood in downtown Toronto.

He was also indicted yesterday over the deaths of a man who went missing in 2012, another whose disappearance was reported almost three years ago, and a homeless man who was believed to have been murdered in 2016 or 2017.

Two of the victims did not fit the profile of some of the others - men of Middle Eastern descent known to have frequented Toronto's gay village neighbourhood - suggesting the accused's crimes may extend beyond that district.

The investigation "certainly encompasses more than the gay community. It encompasses the city of Toronto," Mr Idsinga said.

Mr McArthur, however, was known to have had a long-term sexual relationship with one of the men he has been charged with murdering, police said.

Although Mr McArthur had come under suspicion last September in connection with the disappearance of one man, police rejected suggestions in December that a serial killer was prowling Toronto's gay neighbourhood.

Authorities are now reviewing missing persons cases dating back to at least 2010, and asking anyone who employed Mr McArthur.