Iranian media say a metro tunnel that was under construction in Tehran has collapsed, killing at least three workers.

The semi-official Fars news agency and other outlets say 16 workers survived Wednesday's cave-in, including six who were taken to the hospital for treatment. Rescuers are still searching for one worker.

Tehran's underground metro has five operating lines totaling 180 kilometers (112 miles), and there are plans to add two more by 2018. The line passing through the collapsed tunnel is not in use.