Two people are missing and four are in hospital following an explosion at a steelworks.
Emergency crews were called to the Celsa Manufacturing plant in Cardiff amid reports of an industrial accident.
Eyewitnesses said a loud bang shook nearby buildings before thick black smoke billowed into the air.
A South Wales Police spokesman said: "A fire at the steelworks has been extinguished and a multi-agency operation is ongoing to locate two people who are currently unaccounted for.
"Five people were injured. Of these, one was treated and discharged at scene and four were taken to the University Hospital of Wales, Cardiff.
"The incident is contained to the site, which is being searched."
The alarm was raised just after 10.30am this morning and more than two dozen ambulances and fire engines were deployed, along with the air ambulance.
One man working at a nearby site said: "There was a massive bang. We all felt the floor shake. It was very worrying seeing lots of ambulances and fire engines go in there."
While investigations into the incident are still under way, steelworkers' group Community Union said it believed the blast came from the bar and rod mill at the site and may have involved an oil accumulator in the basement of the building.
A spokesman added: "Our immediate thoughts are with the individuals affected and their families.
"A member of our regional team is on his way to the site now to see what assistance we can offer and to seek further clarity on what has happened."
Cardiff South and Penarth Labour MP Stephen Doughty said the news came as huge shock to the local community.
He said: "I have spoken to senior management at the plant and clearly the situation is ongoing, with a number of reports of injuries.
"My thoughts are of course with the workers caught up in this incident, and their families, and the emergency services responding at the scene."
That sentiment was also expressed by several other politicians - including Welsh Secretary Stephen Crabb, who spoke about the matter during a debate in the House of Commons.
The Health and Safety Executive (HSE) said it was aware of the incident and one of its inspectors was at the site - though the investigation at this stage was being led by the police.
Health officials warned that, while the University Hospital of Wales's "major incident protocol" had now ended, there could stillbe delays for non-urgent patients.
Alice Casey, chief operating officer for Cardiff and Vale University Health Board, said: "We received four casualties from the incident and they are currently being assessed and treated by the trauma team at the emergency unit at University Hospital of Wales.
"We would like to thank our staff for their professional and efficient response to this incident and our partners and the public for their support."