AT&T was hit by a widespread outage that affected users across the United States early Thursday and gave no indication of when services would be restored.

The outage, which affected people in cities including Atlanta, Los Angeles and Dallas, was first reported around 3:30 a.m. ET, and problems with cell service and Internet were still being reported five hours later, according to Downdetector.comwhich tracks user reports of telecommunications and Internet outages.

AT&T said in a statement Thursday that some of its customers were experiencing wireless service interruptions. “We are working urgently to restore service to them,” the statement said, without giving an estimate of when service would return. “We recommend using Wi-Fi calling until service is restored.”

Around 8 a.m., about 70,000 people reported to Downdetector that they were experiencing problems with AT&T. Verizon and T-Mobile were seeing a much smaller number of reports, around 4,000 and 2,000, respectively.

AT&T said on its website that the outages were affecting users in cities such as San Diego, Richmond and Miami, and the initial cause was listed as “maintenance activity.”

Verizon and T-Mobile said in statements that their networks were operating normally.

“Some customers experienced issues this morning calling or texting customers served by another provider,” Verizon said. “We continue to monitor the situation.”

In an email, T-Mobile said: “We did not experience any outages. Our network is working normally. “Downdetector likely reflects the challenges our customers had when trying to connect with users on other networks.”

San Francisco Fire Department said on social media that it was aware of an issue affecting AT&T users trying to call 911. “We are actively participating and monitoring this,” the fire department said. “If you are an AT&T customer and cannot reach 911, try calling from a landline.”

He city ​​of Upper Arlington, Ohio, said the Fire Department may not be notified of fire alarms due to the outage. He urged that any fire alarm be followed by a call to 911. A flood of AT&T users took to social media to complain about their service or ask if others were experiencing similar problems.

This is a developing story.

Victor Mather contributed reports.