State health and fishery officials are extending the closure of several oyster beds in Boston’s South Shore area because of illnesses linked to seasonal shellfish bacteria.

The beds in Duxbury Bay, Kingston Bay, Bluefish River, Back River and Plymouth Harbor have been under a 7-day precautionary closure since last week. Tuesday’s announcement extends that until Oct. 8. It could be extended again if more illnesses arise.

The Massachusetts Department of Health and the state Division of Marine Fisheries say six illnesses linked to contaminated oysters have been confirmed. The illnesses are blamed on the Vibrio bacteria that thrive in warmer ocean waters.