Pakistan Energy Minister Omar Ayub said that a massive power breakdown just hit the country before Saturday midnight, reported Dawn.

The blackout was first reported on social media by residents of major urban cities such as Islamabad, Lahore, Karachi and Multan. Later, those in other cities and towns across the country also complained of power outage.

In a series of tweets, Ayub said that the power outage was caused after the frequency in the power distribution system suddenly fell from 50 to zero. “We are trying to ascertain what caused the drop in frequency,” he said, according to Geo TV. “People are urged to remain patient,” the minister said in a tweet.

The minister said attempts were being made to fire up the Tarbela power station which will lead to a sequential restoration of power supply. “All of our teams have reached their respective stations,” Ayub said. “As the minister for power I am personally overseeing the work for restoration of power. We will keep you updated periodically over the progress in power restoration.”

In another tweet, Ayub said that three units of the Tarbela power house and some of the Warsak power house have been turned on. “The transmission frequency is being matched to the transmission system,” he added.

Following Ayub’s tweets, Pakistan’s Ministry of Energy said the initial reports show that the Guddu power plant developed a fault at 11.41 pm on Saturday night. “The fault caused the country’s high transmission lines to trip, which in turn caused the system frequency to drop from 50 to 0 in less than a second,” the ministry tweeted. “The drop in frequency caused power plants to shut down.”

It said that phase-wise restoration of power will be initiated soon. “Once the initial frequency is met, the restoration work speeds up,” the ministry said.

The breakdown also affected Pakistan-Occupied Kashmir. According to Geo News, 29 districts of Balochistan were also without power. Mobile and internet services were also affected because of the breakdown.

In 2015, an apparent rebel attack on a key power line had plunged about 80% of the country into darkness, according to AFP. The blackout, considered one of the worst in Pakistan’s history, caused electricity to be cut in major cities, including Islamabad, and also affected one of the country’s international airports.