Beijing on Sunday accused the United States of carrying out cyberattacks on China's national timekeeping centre in Xi'an.
The Ministry of State Security accused the US National Security Agency (NSA) of mounting the attacks, beginning in March 2022.
The ministry said the attacks had started by exploiting security loopholes in staff mobile phones, followed by an attack on computers in the centre.
The centre is responsible for generating and maintaining official time in China. This forms the basis for communications networks, financial systems and electricity supply.
Attacks of this kind could cause serious disruption, the report said. China has one time zone, despite its size.
There was no initial reaction from the US. Both sides regularly accuse each other of cyberattacks.
