A whale-watching boat has sunk off the coast of British Columbia in western Canada, leaving at least five people dead.

The boat carrying 27 people sank near Tofino on Vancouver Island, the coastguard said. Sea conditions at the time were reported to be calm.

Emergency officials said 21 people had been rescued and one other person was still missing.

The nationalities of the victims are not yet known.

The boat, Leviathan II, made a mayday call late on Sunday afternoon, the Joint Rescue Co-ordination Centre said.

Valerie Wilson, of the Vancouver Island Health Authority, told Associated Press that the survivors had been admitted to Tofino General Hospital and three of them had since been transferred elsewhere.

The first rescue boats to arrive belonged to the nearby Ahoushat First Nation community, aboriginal councillor Tom Campbell said.

He said he saw the first survivors brought ashore.

"Their looks tell the whole story,'' he told AP. "You can't describe looks on people that are lost. They look totally lost - shocked and lost.''

Canadian broadcaster CBC said the boat appeared to have sunk some distance from the shore.

The 20m (65ft) boat was operated by Jamie's Whaling Station and Adventure Centres.

In a statement on the company's website, owner Jamie Bray said: "It has been a tragic day. Our entire team is heartbroken over this incident.

"We are doing everything we can to assist our passengers and staff through this difficult time. We are co-operating with investigators to determine exactly what happened."

Tofino is a popular destination for tourists wanting to spot humpback and Pacific Gray whales, and trips usually last up to three hours.