The blast damaged the pipeline near the town of Dera Murad Jamali in Baluchistan province, shutting supply to the 586-megwatt Uch power plant.
Abdul Samad Lasi, the area's senior administrator, said: "The power plant has been closed down."
He could not say how long repairs would take and officials from the power plant were not immediately available.
The militants, who are fighting for more autonomy and benefits from oil and gas exploration in Baluchistan, have attacked the pipeline four times in the past month in their campaign against central rule.
The main shareholders of the plant are International Power, a British company, and Tenaska Inc and GE Capital, which are both American. The plant sells electricity to Pakistan's state-run Water and Power Development Authority.
Violence has surged in Baluchistan since December and 21 people died over the weekend, including at least 12 in a bus bombing.
The violence has posed another challenge to efforts by Pervez Musharraf, the president, to bring stability to a nation already troubled by Islamist militancy.
Pakistan's military increased their actions against militants in Baluchistan after a rocket attack on 14 December during a visit by Musharraf. The crackdown coincided with the announcement of plans to privatise two gas distribution firms in the province.
Baluch nationalists say hundreds of people have been killed. Analysts say this could be an exaggeration, but the independent Human Rights Commission of Pakistan has accused the government of gross human rights violations in the province.