AT&T was hit by a widespread outage affecting 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. Eastern time, and cellular service and internet problems were still being widely reported more than five hours later, according to Downdetector.com, which tracks user reports of telecommunication and internet disruptions.

AT&T said in a statement on Thursday that some of its customers were experiencing wireless service interruptions. “We are working urgently to restore service to them,” the statement said. “We encourage the use of Wi-Fi calling until service is restored.”

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

By 10 a.m., about 65,000 people were reporting to Downdetector that they were experiencing problems with AT&T, down from a peak of more than 70,000 at 9 a.m. Verizon and T-Mobile were seeing much smaller numbers of reports, at about 3,000 and 2,000, respectively. Earlier, AT&T’s website had showed outages across the country, including in 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 when calling or texting with customers served by another carrier,” Verizon said. “We are continuing to monitor the situation.”

In an email, T-Mobile said: “We did not experience an outage. Our network is operating normally. Downdetector is likely reflecting challenges our customers were having attempting to connect to users on other networks.”

Cricket, which is owned by AT&T, also reported that its users were experiencing wireless service interruptions and said it was working to restore service.

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

Cities urged citizens to find alternate ways of reaching emergency or municipal services, like landlines or phones connected to Wi-Fi. The city of Upper Arlington, Ohio, said the Fire Department might not be notified of fire alarms because of the outage. It urged that any fire alarm be followed up with a 911 call.

The Massachusetts State Police said on social media on Thursday morning that 911 call centers across the state had been flooded with calls from people checking to see if 911 worked from their phones. “Please do not do this,” the police said. “If you can successfully place a non-emergency call to another number via your cell service then your 911 service will also work.”

This is a developing story.

Victor Mather contributed reporting.