Five hours after a fire at an electrical substation knocked out power for Heathrow Airport and thousands of homes in western London, the city’s fire brigade said early Friday that it was still trying to bring the blaze under control.

By 4 a.m. Friday, half of a transformer remained on fire, the London Fire Brigade said, warning that the disruptions caused by the blaze would increase into the morning.

The blaze at the North Hyde substation, around two miles north of Heathrow, left more than 16,000 homes without power, according to the regional power utility, Scottish & Southern Electricity Networks.

“This will be a prolonged incident, with crews remaining on scene throughout the night,” Pat Goulbourne, assistant commissioner at the London Fire Brigade, said in a statement. “As we head into the morning, disruption is expected to increase, and we urge people to avoid the area wherever possible.”

The London Fire Brigade said it had deployed around 70 firefighters to the substation, where a transformer was ablaze. Thick smoke was seen rising from the burning substation in a photo posted on social media by the fire brigade.

Around 150 people were evacuated, and the authorities set up a 650-foot cordon around the substation as they tried to bring the fire under control.

The cause of the fire was not yet known, the fire brigade said.