A gas pipeline was blown up in Sindh late on Tuesday, the latest attack in an area where tribal militants are waging a renewed campaign for autonomy.
The pipeline feeds gas to Guddu power station, the country's biggest thermal power station, in Sindh.
A spokesman for Balochistan Liberation Army fighting for more autonomy in neighbouring Balochistan called newspaper offices to claim responsibility for the attack.
"We have blown up this pipeline. We will continue such attacks until our demands are met," Azad Baloch, a spokesman for Balochistan Liberation Army, said in a telephone call to a newspaper office in Quetta.
The disruption will cause a shortfall of 250 megawatts of electricity per day, according to Water and Power Development Authority (Wapda).
"We will not suspend power supplies to any area and we will make up the deficiency from other power stations," Chaudhry Mohammad Khalid, a senior official of Wapda told private television.