A string of bushfires is burning across South Australia sparked by extreme conditions, with temperatures surging into the high 30s in many districts.

The Country Fire Service (CFS) said eight fires were active on Wednesday afternoon, with the most serious at Auldana, in the Adelaide Hills and near Roseworthy, north of Adelaide.

They posed a threat to homes, the CFS said.

The Auldana blaze was burning out of control in scrub near Skye Cellars.

The Roseworthy fire was also burning out of control.

Weather conditions across SA were expected to become moderate by Thursday, with temperatures falling back into the low 20s.