Underscoring Cambodia’s heavy dependence on imported electricity, the lights went out not just in Phnom Penh but across provinces nationwide on Thursday, an unprecedented blackout that Prime Minister Hun Sen said was caused by an electrical fault in neighbouring Vietnam.

Between about 10pm and 11pm, Phnom Penh, Siem Reap, Battambang, Preah Sihanouk and several other provinces lost power, just as revellers in the capital were enjoying the last hours of Water Festival.

“As the government’s leader, I am very sorry and would like to confirm to citizens that the power outage is not the sign of a terrorist attack or any other incident besides the electrical outage,” the premier said, in a speech broadcast on his Facebook soon after the blackout.

Hun Sen said though Cambodia has a contract allowing it to buy 200 megawatts from Vietnam, it was currently using just 150 megawatts of that to power Phnom Penn and other provinces.

“When the electricity in Vietnam was disconnected, it was disconnected here too,” he said, adding that about 50 per cent of imported power was consumed by the capital.

According to state energy provider Electricite du Cambodge (EdC) president Keo Rattanak, the blackout also affected Banteay Meanchey, Kampong Chhnang, Pursat, Kampong Cham, Kampot and Kampong Thom provinces.

Rattanak said EdC engineers worked all night to restore power, though some outages still remained until yesterday morning. He said his Vietnamese counterparts had traced the fault to a 230-kilovolt transmission line.

Rattanak said in addition to the 150 megawatts from Vietnam, Cambodia had the capacity to generate at least 350 megawatts of power domestically to meet energy demands.

Though it is focusing on developing hydropower plants and last year opened its first coal-fired power station, Cambodia relies heavily on imported fuel and electricity for energy security, and its electricity costs are among the highest in the world.

According to the latest report from the Electricity Authority of Cambodia, the country used more than 3 billion kilowatt hours of electricity in 2014, with more than half imported from neighbouring countries, including about 1.3 billion kilowatt hours from Vietnam.

San Vibol, an energy researcher at the Royal University of Phnom Penh, said he was surprised at how severely a disruption of supply from Vietnam affected Cambodia’s power grid, adding that it was a reminder that more needed to be done to ensure the country’s energy security.

“Many countries import their power from other countries … the problem is how reliable it is. At the moment, it is not stable. Just look at last night, a black night in Cambodia.

“It’s the second time [since a 2013 Phnom Penh blackout]. The government should think about that, it can cause a lot of problems if we are going to face this again and again.”