A Chinese hack compromised even more US telecoms than previously known, including Charter Communications, Consolidated Communications and Windstream.
Hackers also exploited unpatched network devices from security vendor Fortinet and compromised large network routers from Cisco, The Wall Street Journal reported.
In addition to deep intrusions into AT&T and Verizon, hackers pierced other networks belonging to Lumen Technologies and T-Mobile, according to the report.
China denied engaging in such actions and accused the United States of peddling disinformation.
There is growing concern about the size and scope of the reported Chinese hacking into US telecommunications networks and questions about when companies and the government will be able to assure Americans about the issue.
The report added that US national security adviser Jake Sullivan told telecommunications and technology executives at a secret White House meeting last year that Chinese hackers had gained the ability to shut down dozens of US ports, power grids and other infrastructure targets at will.
The Chinese-linked Salt Typhoon cyberespionage operation targeted AT&T and Verizon's systems, but the wireless carriers' US networks are now secure as they work with law enforcement and government officials, the companies said last week in their first acknowledgment of the attacks.
Lumen said it no longer sees evidence of the attackers in its network and that no customer data was accessed. T-Mobile said it stopped recent attempts to infiltrate its systems from advancing and protected sensitive customer information from being accessed, according to the Journal.
Verizon said a small number of high-profile customers in government and politics were specifically targeted by a threat actor and that those people had been notified.
Cisco and Fortinet declined to comment to the newspaper.
Targets of Salt Typhoon have previously reportedly included officials connected to Democrat Kamala Harris and Republican Donald Trump's presidential campaigns.
