A fire aboard a South Korean Navy submarine undergoing maintenance has left one person missing, prompting an emergency response and ongoing rescue efforts.

Rescue operations resumed Tuesday for four workers trapped under the debris of a collapsed boiler tower at a thermal power plant in Ulsan, southeastern South Korea, after two neighboring towers were safely demolished.

At least six people are believed to be trapped after a large structure collapsed at the Korea East-West Power’s Ulsan Power Plant headquarters on Thursday.

An oil storage tank in the South Korean southeastern city of Ulsan exploded Monday, leaving one person dead and another injured, officials said.