A Thai cargo ship thought to be carrying up to 500 Tamil refugees from Sri Lanka is approaching Canada's Pacific coast, officials said.
The MV Sun Sea is being tracked by a Canadian warship and will be escorted to a port, they added.
Public Safety Minister Vic Toews has said there is reason to believe members of the defeated Tamil Tigers movement are on the ship.
The Tamil Tigers have been outlawed in Canada as a terrorist group since 2006.
The ship has entered Canada's "exclusive economic zone", an area that stretches 200 nautical miles (370km) from the coast, the government said.
Canadian territorial waters extend roughly 12 nautical miles (22km) off the coast.
The ship will probably be escorted to Victoria, British Columbia, arriving late on Thursday or early on Friday, authorities said.
Initial reports said the cargo ship had about 200 Tamils on board.
However, on Wednesday officials in British Columbia said they understood there could be up to 500.
Speaking in Toronto on Monday, Mr Toews said: "I can assure you that we are concerned about who is on that ship and why they might be coming."
Officials anticipate the immigrants will be taken to two correctional facilities in Maple Ridge, a town east of Vancouver.
British Columbia's Ministry of Children and Family Development is likely to take care of any children aboard the ship.
Corrections officers and sheriffs at the prisons have been told to be ready for up to 500 Tamil migrants or refugees in the coming days, most of them men, the Associated Press news agency reported.
The US Coast Guard is also reported to be monitoring the ship.
The Tamil Tigers were defeated by the Sri Lankan army last year after more than two decades of conflict.