A union on Wednesday urged the government to enhance monitoring of construction companies, saying it has received complaints about exploitation of imported workers.

At a press briefing, the Construction Industry Employees General Union said it believes that more than 100 such workers are being exploited.

It said some have been told to work extra shifts for no pay, and to pay for training fees and the tools they use.

The union alleged that a company involved in the airport's third runway project told workers to hand over their ATM cards and confiscated more than half of their salaries.

The union's rights and complaints officer, Ng Wai-leung, said the complaints are only the tip of the iceberg and it's likely that many abused workers are keeping silent.

"They don't want to be blacklisted by their company, making them not able to come to Hong Kong again. This is their main concern," Ng said.

He added that some workers have been sacked and sent out of Hong Kong after seeking help from the union.