BAKU, Azerbaijan (AP) — Azerbaijan has been hit by a massive blackout affecting most of the country, the worst power outage since the 1991 collapse of the Soviet Union.

President Ilham Aliyev set up a government commission to investigate the accident at a power plant in Mingechavir that caused the blackout early Tuesday. The Emergencies Ministry said a transformer's breakdown in Mingechavir sparked a fire that was put out in 20 minutes and inflicted no casualties.

The blackout came amid a heat wave in the Caspian Sea nation, with temperatures exceeding 40 degrees Celsius (104 degrees Fahrenheit) resulting in a power consumption surge that plunged the capital, Baku, and nearly 40 other cities and regions into pitch darkness.

It took several hours to fully restore power in Baku but efforts to restore power in other regions continued.