China’s top intelligence agency on Sunday accused the U.S. National Security Agency of carrying out a yearslong cyberespionage campaign against Beijing’s national time-service network, among the nation’s most sensitive pieces of digital infrastructure.

China’s Ministry of State Security said the alleged intrusion began in early 2022 when NSA targeted the National Time Service Center, which keeps and broadcasts China’s standard time, including to telecommunications, finance, transportation, and defense organizations. 

Investigators allege the hackers deployed 42 “specialized cyberattack weapons” to maintain persistence and extract network data. An overseas phone provider was exploited to gain initial access, China said, without naming the specific company. From there, U.S. cyberspies were able to access staff members’ mobile devices and other timekeeping systems, China further alleged.

“NSA does not confirm nor deny allegations in the media regarding its operations,” an NSA official told Nextgov/FCW. “Our core focus is countering foreign malign activities persistently targeting American interests, and we will continue to defend against adversaries wishing to threaten us.”

The NSA is an intelligence agency under the Department of Defense that employs various hacking, codebreaking, and eavesdropping capabilities to gather data on adversaries around the world. 

In a media statement, the U.S. embassy in Beijing said that China “is the most active and persistent cyber threat to U.S. government, private-sector and critical infrastructure networks.” It did not address the specific accusations.

The Chinese intelligence agency has not released forensic samples or indicators of compromise related to the National Time Service Center. It said the U.S. has “repeatedly hyped up the ‘China cyber threat’ theory, coercing other countries to hype up so-called ‘Chinese hacker attacks,’ sanctioning Chinese companies and prosecuting Chinese citizens in an effort to confuse the public and distort the truth.”

The claims illustrate years of ongoing cyber tit-for-tat between Washington and Beijing, advanced nations capable of deploying cyber capabilities against each other at any given time. 

In recent years, China has breached troves of telecommunications networks and other critical infrastructure systems in the U.S. and around the world. Around a decade ago, documents revealed by former NSA contractor Edward Snowden showed the spy agency had infiltrated the networks of Huawei, a major Chinese telecom operator. In April, Chinese authorities accused the NSA of launching attacks against networks tied to the Asian Winter Games that were held in February.