A worker climbs a cell tower in Oakland, California.

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A cell network outage early Thursday affected thousands AT&T users across the United States, cutting off calls and text messages, as well as 911 services in major cities, including San Francisco.

About 70,000 incidents were reported as of 10:00 a.m. ET, according to data from the outage-tracking website Downdetector.com. The reason for Thursday’s outage was not immediately clear.

“Some of our customers are experiencing outages in wireless service this morning,” AT&T said in a statement to CNBC. “Our network teams took immediate action and so far three quarters of our network has been restored. We are working as quickly as possible to restore service to remaining customers.”

A spike in outages began around 4:00 a.m. ET and peaked at about 74,000 reported incidents at 8:30 a.m. ET, according to Downdetector.

Shares of AT&T fell about 2% Thursday morning after the outages.

The AT&T outage affected people’s ability to contact emergency services by dialing 911, the San Francisco Fire Department said in a post on social media platform X.

“We are aware of an issue affecting AT&T wireless customers making and receiving all phone calls (including to 911),” the fire department said on the platform.

Atlanta Mayor Andre Dickens said in an X post that the city can receive and make outgoing 911 calls, but that AT&T customers in the area have reported problems.

“We have received calls from AT&T customers that their cell phones are in SOS mode. Please direct all service restoration inquiries to AT&T,” Dickens said.

Massachusetts State Police said people have been flooding their 911 center with calls trying to determine if the service is working from their cellphones.

“Please don’t do this. If you can successfully make a non-emergency call to another number through your cellular service, then your 911 service will also work,” state police said in a post on X.

Users of Verizon and T-Mobile were reporting several thousand outages each as of 10:00 a.m. ET, according to Downdetector.

Those reports are likely due to calls made while trying to connect to other networks, both companies said.

“Downdetector likely reflects challenges our customers have faced trying to connect to users on other networks,” T-Mobile said in an emailed statement.

— Reuters and CNBC’s Steven Kopack contributed to this report.