Updated below:

The largest telecom provider in Ukraine today was hit with a crippling cyberattack, presumably from a Russian source, that left millions of people without cell and internet services.

Kyivstar, which has 24 million subscribers, reported via its Facebook page that it got hit by a powerful cyberattack that led to a “large-scale technical failure,” according to machine translations of the page. As a result of the attack, it completely shuttered its entire network to isolate the cause of the attack.

Its parent company, the Dutch internet operator Veon Ltd., confirmed the outage and said it’s working to determine the cause and to mitigate future attacks with increased network security measures. Ukrainian’s security service has opened a criminal investigation and is working with their cyber specialists to restore service. Subscriber personal data has not been compromised, according to various sources.

The outage prompted many Kyivstar customers to purchase new prepaid SIM cards and switch their cell service to another provider. This switch is relatively simple, without the need to purchase a long-term contract.

However, the competing providers, Vodafone and Lifecell, saw huge jumps in customer connections, which resulted in network congestion with higher loads and slower connection speeds. Lifecell reported to The Record that some of its apps were temporarily down because of increased demand.

The Kyivstar outage has had repercussions with two Ukrainian banks, PrivatBank and Monobank. The former’s banking network had ATMs and point-of-sale terminals go offline because of the outage. The latter bank coincidentally was hit with a denial-of-service attack that was quickly mediated and services restored today, according to its Telegram channel.

Update: Veon provided an update saying “as of 8 p.m. Kyiv time today, it has partially restored the operation of fixed-line services.” It plans to restore more service tomorrow morning.

“This hack has some military implications: firstly a lot of air raid warning systems are now offline, and secondly a lot of Ukrainian military communications is done over mobile phone,” The Grugg, a prolific security blogger, wrote in an email to SiliconANGLE . “I also think drone operators will have less bandwidth to communicate with artillery and other support elements. This will decrease their operational capacity and reduce their defensive capabilities. It is damn hard to run a modern war without a data link.”

Update #2: As of Thursday, most of the network services have been restored, according to Veon. “More than 90% of mobile base stations are now operational and voice services have been switched on across the country,” it wrote in an email.

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