Pakistan was plunged into darkness after a breakdown of a key power transmission line in southern part of the country, another reminder of the country's crippling energy crisis.

The power failure on Sunday, one of the worst in the country has experienced, has caused power to be cut in major cities throughout the country including the capital Islamabad.

The blackout began when a 500 kva transmission line, carrying electricity from the private sector Hubco Power Plant to the national grid, tripped.

The country's state minister for water and power apologised for inconvenience and said work was under process to restore electricity.

"On the Prime Minister's directive, we are not to sleep till this problem is resolved," he said in a message on Twitter.

A senior official at the National Grid station in Islamabad said it will take about eight hours to restore electricity to the country.

"Around 80 percent of the country is under the spell of the power breakdown, it will take around eight hours to restore electricity," the official told AFP on conditions of anonymity.

Pakistan's electricity distribution system is a complex, and delicate, web and a major fault at one section often leads to chain reaction and breakdowns of power generation and transmission.

In addition to chronic infrastructure problems, the energy sector is also trapped into a vicious "circular debt" brought on by the dual effect of the government setting low electricity prices and customers failing to pay for it.

State utilities therefore lose money, and cannot pay private power generating companies, which in turn cannot pay the oil and gas suppliers, who cut off the supply.

Earlier this week, Prime Minister Nawaz Sharif cancelled his trip to the World Economic Forum in Davos to deal with a severe petrol shortage at home.

The fuel crisis began last week when Pakistan State Oil was forced to slash imports because banks refused to extend any more credit to the government-owned company, which supplies 80 percent of the country's oil.

Solving Pakistan's energy crisis was a key campaign pledge for Sharif in the run-up to the 2013 general election, and the shortage is heaping fresh pressure on his government.

Source: Agencies