Canadian officials have boarded a cargo ship thought to be carrying about 500 Tamil migrants from Sri Lanka.
The authorities intercepted the MV Sun Sea off British Columbia. They say the migrants will now be vetted amid fears that some are Tamil Tiger rebels - banned by Canada as a terrorist group.
The ship reportedly crossed the Pacific after Australia turned it away.
The Tamil Tigers were routed last year by the government, which has been widely accused of rights abuses.
Sri Lanka's high commissioner to Canada, Chitranganee Wagiswara, has urged Canada to refuse their asylum claims.
She says the ship is part of a people-trafficking operation linked to the Tamil Tigers.
Canadian Public Safety Minister Vic Toews said the vessel had declared it had 490 refugees on board, but he said some were "suspected human smugglers and terrorists".
Several Canadian vessels have been guiding the Sun Sea towards Vancouver Island, where tents have been set up and the migrants will be processed.
Officials said those on board were being offered food and water.
Toronto-based immigration lawyer Lorne Waldman told the Canadian Broadcasting Corporation (CBC) that the country had "no choice" but to take the people in.
"Once they're here, we can determine what we'll ultimately do with them," he said.
Last October, a ship carrying 76 Sri Lankan migrants was intercepted in Canadian waters after crossing the Pacific.
All of the men were immediately detained in jails around the Vancouver area, but all apart from one were released shortly afterwards.
Some of those on board said they paid thousands of dollars for a berth.
The Tamil Tigers were defeated by the Sri Lankan army last year after more than two decades of conflict.
The rebels became notorious for their use of child soldiers, suicide bombers and human shields.
But government forces were criticised for the bloodshed that marked the end of the war, after which many Tamils were rounded up into camps.