China on Friday rejected accusations it was behind a hacking attack that saw data on up to 80 million customers stolen from US health insurance giant Anthem as "groundless".
The Bloomberg News agency cited three people with knowledge of Anthem's investigation as saying cybersleuths believed the breach bore the hallmarks of previous attacks blamed on Chinese hackers.
"The US side should not make groundless accusations against China," foreign ministry spokesman Hong Lei said at a regular briefing in Beijing.
"It is unreasonable to make an accusation without enough evidence."
The cyberattack is the latest where US investigators say evidence points to China. FBI director James Comey last October said it was at the "top of the list" of countries launching cyberattacks on US firms.
Last year, five members of a Chinese army hacking team known as Unit 61398 were indicted by US federal prosecutors on charges of stealing information from companies, including nuclear plant manufacturer Westinghouse, SolarWorld and US Steel.
"It is very difficult to determine the source of hacking activities, especially when it is carried out across borders," Hong said.
The information stolen from Anthem includes names, birth dates, social security numbers, street addresses, email addresses and employment information, the company said.
Bloomberg and The Wall Street Journal reported that while the investigation into the attack was in its early stages, there were indications it could be part of a broader spying campaign rather than profit-driven identity theft.
With details about a person's medical records, for example, cyber spies could craft emails that appeared legitimate to business or government agency workers but were rigged with malicious software to gain access to their employers' computers networks.