An emergency warning is in place for several towns north of Adelaide as a bushfire continues to burn, but the Country Fire Service says the spread of the blaze has been slowed.

The fire is threatening the towns of Roseworthy, Sandy Creek, Freeling and Lyndoch as it burns in grassland and crops.

The CFS says crews are working to complete control lines around the fire and motorists have been asked to avoid the area.

It comes as temperatures across SA soared on Friday, with the mercury hitting 40C in Adelaide and pushing into the mid-40s in some regional centres.

Gusty winds are also causing concern for firefighters.

The CFS declared an extreme bushfire risk for the Adelaide Hills on Friday.

It said the risk would be severe in six other districts.

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Similarly hot conditions will continue into Saturday, with Adelaide to have a top of 38C, before a cooler, southerly change pushes through.

AAP

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