New Delhi, Nov. 12 -- A massive power outage plunged the entire Dominican Republic into darkness on Tuesday, disrupting traffic and halting businesses across the nation of nearly 11 million. Officials said the blackout stemmed from a failure in the transmission system, though the exact cause remains unclear.

Generation units at San Pedro de Macoris and the Quisqueya Power Plant shut down, setting off a chain reaction of failures across other transmission and generation facilities, the Dominican Electricity Transmission Company said.

It wasn't immediately clear how quickly power would be restored, but as of Tuesday night, some 15% of the system was back online, according to Energy Minister Joel Santos.

"We'll restore power little by lit...