Two construction workers were killed and 15 were injured when a nearly completed office block in Roodepoort collapsed on Thursday, Johannesburg emergency management services said.

When the building on Van Staden road collapsed at 8.15am on Thursday there were 40 people working on site, said spokesperson Percy Morokane.

About half of the structure collapsed and the parts still standing were declared unsafe.

One person was critically injured and was taken to the Chris Hani Baragwanath hospital. Fourteen other people were seriously injured and were taken to Helen Joseph and Leratong hospitals.

Twenty-eight urban search-and-rescue technicians were sifting through the rubble to locate one of the bodies. Police dog units were helping with the search.

Structural engineers were on site to assess damage and to advise on rescue operations.

Representatives from the Department of Labour were expected on scene shortly as well as officials from the city's building control section, Morokane said.