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AT&T has restored access to its mobile phone customers after widespread outages Thursday, the company said in a statement.

Tens of thousands of cell phone users across the US reported outages earlier in the day.

Downdetector.com, which tracks outages, showed reports indicating no service or no signal after 04:00 EST (09:00 GMT).

AT&T apologised to customers and said services had been fully restored by early afternoon.

"We are taking steps to ensure our customers do not experience this again in the future," the company said in a statement.

The cause of the outage is under investigation.

Other carriers including Verizon and T-Mobile told the ´óÏó´«Ã½ that their networks were operating as normal.

But both carriers said customers had likely logged service issues after making unsuccessful attempts to contact users of another provider.

AT&T customer complaints peaked at just over 74,000, with hotspots in the south and east,

Cricket Wireless, a small carrier, also reported service interruptions, as did customers with two other small carriers, UScellular and Consumer Cellular.

The complaints included issues with calls, texts and internet access, with most reports indicating no service or no signal.

Downdetector's heatmap showed major population centres including Los Angeles, Chicago, Houston and Atlanta were among those facing the highest number of outages.

Some also reported experiencing difficulties with 911 service, with officials across the country advising the use of a landline, social media or a cell phone with a different carrier in the event of an emergency.

The US government is concerned about the widespread outage, with the FBI and Department of Homeland Security both investigating what happened, US National Security Council spokesperson John Kirby told a press briefing.

The US Cybersecurity and Infrastructure Security Agency is "working closely with AT&T to understand the cause of the outage and its impacts, and stand ready to offer any assistance needed", said Eric Goldstein, its executive assistant director for cybersecurity, in a statement.

Citing a confidential memo from earlier Thursday, ABC News reported that CISA officials believe "there are no indications of malicious activity".