OSLO (Reuters) - Sweden's top nuclear power generators have been forced to cut output because of exceptionally warm weather in Scandinavia, and their output could be reduced for over a week, their operators said on Wednesday.

Oskarshamn, part of Germany's E.ON and Forsmark, operated by Swedish utility Vattenfall have both cut output because warm sea water temperatures are limiting their ability to cool down.

"For each degree above 23 decrees Celsius in the cooling water, each unit has to decrease power by 3 percent," Forsmark said in a market message. "It is uncertain how long this will last, but according to meteorologists, the warm weather will last for at least 11 more days."

Temperatures exceeded 30 degrees in the southern part of Scandinavia this week, hitting their highest level in years.

The Forsmark nuclear power generator has three units with over 3,000 megawatts of built in capacity and data from the plant showed it was operating at about 92 percent of its built in capacity.

If sea temperatures reach 26 degrees, block 3 has to be shut, while at 28 degrees, the other two have to be closed as well, a Vattenfall spokesman said. Sea temperatures are currently around 22-23 degrees, according to the Swedish weather service.

Oskarshamn, which currently operates two blocks with around 1,950 megawatt of built in capacity, has had to reduce output by about 90 megawatts because of warm cooling water temperatures, a spokesman said. Data from the plant showed it was operating at 93 percent of its built in capacity.

Vattenfall's Ringhals plant is not yet affected.


(Reporting by Balazs Koranyi, editing by David Evans)