A massive power outage in Turkey on Tuesday crippled services in over 40 cities, forcing planes to divert and affecting train services, local media reported. The outages on Tuesday led to the cancellation of the Istanbul metro’s services and train lines to the capital of Ankara. A number of social media posts also indicated that flights had been cancelled or delayed.

Prime Minister Ahmet Davutoglu said authorities were investigating all possible causes, and did not rule out the possibility of a terror attack, Hurriyet Daily News reported. Energy Minister Taner Yildiz told media that power had been restored to about 80 percent of all affected areas, Reuters reported, and that electricity would soon "be completely restored."

The country’s energy ministry released a statement saying its main distribution lines were affected, likely originating somewhere in the western Aegean region, Hurriyet reported. It added that the blackout is Turkey’s biggest in 15 years. 

"Our main target right now is to restore the network. This is not an incident that we see frequently," Yildiz had said earlier on Turkish television, according to Reuters. "Whether or not terrorism is a high possibility or a low one I can’t say at this stage. I can’t say either whether it is a cyber attack," he told reporters.

According to the U.S. Energy Information Administration, Turkey’s power consumption has risen rapidly over the past few years, as the country has experienced solid economic growth. It is also a critical region for energy transit, moving oil and gas from Russia, the Caspian region, and from the Middle East to Europe.