* Grid operators say 2 reactors must be brought back online
* Say 2,000 MW needed or winter blackout possible in south
BERLIN, May 22 (Reuters) - Germany's southern regions could be hit by an electricity blackout this winter if seven nuclear power plants taken offline in March remain down, power grid operators have said.
The four operators, in a letter to the economy ministry obtained by Reuters on Sunday, said imported power would not cover the shortfall during the colder months unless some customers were removed from the network.
"An independent supply of customers is in danger," Amprion, Tennet, 50hertz and EnBW-Netze said, adding 2,000 megawatts -- around the capacity of two nuclear reactors -- was required to ensure power supply when winter demand jumped.
Chancellor Angela Merkel has imposed a three-month moratorium on operation of the oldest seven of Germany's 17 nuclear plants, after a massive earthquake and tsunami hit Japanese reactors in March and led to environmental disaster.
Environment minister Norbert Roettgen has signalled the seven plants taken offline will remain so. Merkel's cabinet will decide on the country's nuclear exit strategy on June 6.
At the weekend, only four of Germany's plants were online, due to the moratorium and maintenance operations. Germany's network watchdog agency said power supplies were secure despite the plants' being taken off the grid. (Reporting by Markus Wacket; Writing by Brian Rohan; Editing by Dan Lalor)