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 issues with cell service and internet continued to be widely reported more than five hours later, according 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,” the statement says. “We recommend using Wi-Fi calling until service is restored.”

A spokesperson did not respond to questions about what had caused the outage or when service would be restored. At 10 a.m., AT&T’s website showed that The outages were limited to users in California..

At 10 in the morning, about 65,000 people were reporting to Downdetector that they were experiencing problems with AT&T, down from a peak of over 70,000 at 9am. Verizon and T-Mobile were seeing much smaller numbers of reports, around 3,000 and 2,000, respectively. Previously, AT&T’s website had shown outages across the country, including San Diego, Richmond and Miami, with the initial cause 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.”

AT&T-owned Cricket also reported that its users were experiencing wireless service outages and said it was working to restore service.

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.”

Cities urged citizens to find alternative ways to reach municipal or emergency services, such as landlines or Wi-Fi-connected phones. 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 911 call.

Massachusetts State Police said on social media Thursday morning that 911 call centers across the state were inundated with calls from people checking to see if 911 was working from their phones. “Please don’t do this,” the police said. “If you can successfully make a non-emergency call to another number through your cellular service, then your 911 service will also work.”

This is a developing story.

Victor Mather contributed reports.