Police in Canada have foiled a plot to carry out a mass shooting.

Nova Scotia Royal Canadian Mounted Police said it received a tip-off on Thursday morning that a 19-year-old Canadian man and a 23-year-old woman from Geneva, Illinois, had access to firearms and intended "to go to a public venue in the Halifax region on February 14 with a goal of opening fire to kill citizens, and then themselves", commanding officer Brian Brennan said.

Authorities have said the plot was not related to Islamic terrorism.

Police released no details about the motive but said they arrested three young suspects. The fourth suspect, the 19-year-old man, was found dead when police entered a home.

Mr Brennan said the American woman was arrested when she arrived at Halifax's airport.

"Had they been able to carry out their intentions the possibility for a large loss of life was definitely there," he said.

Mr Brennan would not say where they plotted to attack.

Police said two other Nova Scotia men, aged 20 and 17, were also involved and investigators were trying to determine their role.

"We believe we have apprehended all known individuals in this matter and have eliminated the threat. We are not seeking any further suspects at this time in relation to this investigation," Mr Brennan said.

Public safety minister Steven Blaney praised the work of the police in Canada and Illinois, and well as border staff.