Workers at the Voisey's Bay nickel mine in northern Labrador walked off the job Wednesday, forcing the mine to shut down.

The workers said they were concerned for their safety after an overnight incident at the mine involving replacements hired to fill in behind about 120 support staff who walked off the job in April. The support staff, who work for two subcontractors, provide a wide range of services ranging from airstrip maintenance to housekeeping.

The Voisey's Bay nickel mine is located in northern Labrador. (CBC)

Curtis Saunders, the vice-president of the local United Steelworkers local, said employees are refusing to go back to work if the replacement workers are not removed from the site.

Saunders said tensions between the two sides have deteriorated, with replacement workers throwing rocks at regular employees.

On Tuesday, he said, a group of five unionized employees were trapped in a large container by a replacement worker.

"They barred the sea can up with our members in it," Saunders told CBC News.

He said a manager released the penned-in staff, who were reported to be safe.

Voisey's Bay Nickel said a replacement worker was removed from the mine site overnight.

Officials said they were looking into the confrontation and promised to share their findings with employees.

Saunders said the workers want more action than that.

"Everything is completely shut down until they resolve this issue, until they can guarantee the scabs [are] removed from site," he said.

Voisey's Bay Nickel said it is not considering removing any other replacement workers.