T-Mobile’s network was among the systems hacked in a damaging Chinese cyber espionage operation that gained entry into several US and international telecommunications companies. The Wall Street Journal reported, citing people familiar with the matter.

Hackers linked to a Chinese intelligence agency managed to breach T-Mobile as part of a month-long campaign to spy on the cellphone communications of high-value intelligence targets, the Journal added, without specifying when the attack took place .

“T-Mobile is closely monitoring this industry-wide attack,” a company spokesperson said Reuters in an email.

“At this time, T-Mobile’s systems and data have not been significantly affected, and we have no evidence of an impact on customer information.”

It was unclear what information, if any, was taken about T-Mobile customers’ calls and communications records, according to WSJ report.

Last week, the Federal Bureau of Investigation (FBI) and US cyber watchdog CISA said hackers linked to China intercepted surveillance data intended for US law enforcement agencies after breaking into an unspecified number of telecommunications companies .

In early October, the Journal reported that Chinese hackers accessed the networks of U.S. broadband providers, including Verizon Communications, AT&T and Lumen Technologies, and obtained information from systems the U.S. federal government uses for court-authorized wiretapping .

Beijing has previously denied claims by the US government and others that it used hackers to break into foreign computer systems.