A top US Senator has labeled a series of telecom hacks, suspected to be caused by China, as the country's worst-ever telecom hack.

Sen. Mark R. Warner (D-Virginia), chairman of the Senate Intelligence Committee and a former telecom venture capitalist, told the Washington Post, it's the "worst telecom hack in our nation’s history."

According to Warner, the hackers, part of a group called Salt Typhoon, have been able to listen in on audio calls in real-time and have, in some cases, moved from one telecom network to another.

“This is an ongoing effort by China to infiltrate telecom systems around the world, to exfiltrate huge amounts of data,” said Warner.

His comments follow reports earlier this week by Wall Street Journal, that revealed T-Mobile had become the latest carrier to be hacked by China.

The reported attack on T-Mobile follows reports of similar attacks against other US telcos last month including AT&T, Verizon, and Lumen Technologies.

The hack on T-Mobile was reportedly carried out as part of a months-long campaign to spy on the cellphone communications of high-value intelligence targets.

WSJ noted that the hackers were able to infiltrate US telecom infrastructure by exposing vulnerabilities in Cisco Systems routers, while investigators suspect the hackers relied on AI or ML to further their espionage operations, according to people familiar with the matter.

Last week, the Federal Bureau of Investigation (FBI) and the US cyber watchdog agency CISA said China-linked hackers were able to intercept surveillance data intended for American law enforcement agencies.

The FBI, which is investigating the hacks, said that the hackers had targeted government or political activity.