Firefighters rescued five people from the second floor of Auberge 1082, a swingers club on Rosemont Boulevard. (CBC)

Firefighters are still investigating the cause of a fire that forced clients of a swingers club in Montreal to flee in various states of undress Friday morning.

The fire broke out at about 6:30 a.m. ET in the basement of Auberge 1082, located on Rosemont Boulevard near Christophe Colombe Avenue.

"They have the sauna in the basement and rooms on the second floor," said Gilles Ducharme, the Montreal fire department's chief of operations. "It's an adult club. And we had 10 person[s] inside at that moment, three employees and seven customers."

Five of those clients were trapped in rooms on the second floor. CBC reporter Melissa Kent spoke to some who tried to flee but found the exits blocked. Facing clouds of smoke and windows that wouldn't open, patrons tried to make their escape by using a television to break the glass.

Some clients at swingers club Auberge 1082 were forced to flee the burning building in various states of undress. (CBC)

"He threw a TV four times at the window and the TV broke," said one female client who didn't want to be named. Another client said she heard the fire alarm but thought it was a drill and by the time they reacted, the hallways were filled with smoke.

In the end, firefighters were able to break through the window and help clients — some naked, others wearing only towels or robes — escape from the second floor on ladders.

"When they broke down the window, we could barely breathe. We came out, we were in a robe because, you know, it's a swingers' club," said one client.

A neighbour in a nearby apartment, who also refused to be named, witnessed the evacuation from his window and said it was a sight to behold with people running out in towels, half naked in the street.

Six people, including the five on the second floor, suffered minor smoke inhalation.

One bathhouse client said she's grateful to the firefighters.

"I had minor cuts due to the glass and me and three other people needed oxygen, but we're fine. Basically we're fine and we're alive."

An extermination business operates from the main floor of the building. Christina Lyster, who has worked there for the past four years, said she was glad firefighters were also able to rescue four lab rats the business keeps on hand to explain rodent behaviour.

"They're part of our family at the extermination centre. We see them every day, we feed them, so they're part of the group … they're our mascots," said Lyster.

Firefighters said Friday they believe the fire was accidental, likely caused by an electrical problem in the basement sauna.