The extended outage at the Ringhals 4 nuclear power reactor increases the risk of electricity blackouts in southern Sweden this winter, the country's power grid operator said on Tuesday.

"Although it remains small, the risk of effect shortages increases," Pontus de Mare, the head of power system operations at Svenska Kraftnat, told Reuters.

"The price will also probably become very high," he said.

Utility Vattenfall on Tuesday said the restart of Ringhals 4 had been postponed by two months to Jan. 31, putting additional strain on Nordic and Baltic power supply. (Reporting by Anna Ringstrom, editing by Terje Solsvik)