Beijing has said claims that Chinese hackers accessed the personal data of millions of US federal workers are hypothetical and irresponsible. The FBI has launched an investigation.
China has warned the US against "jumping to conclusions and making hypothetical accusations" after Washington announced that data on four million government staff had been hacked.
While officials have refused to assign attribution or motive, one Republican lawmaker, Maine Senator Susan Collins, said the hackers were believed to be based in China.
A Chinese embassy spokesman in Washington said the claims were "not responsible and counterproductive," and that it was hard to track hacker activity across borders.
The US Office of Personnel Management (OPM), which handles security clearances and background checks on potential government employees, said it was notifying staff - past and present - of the breach.
"As a result of the incident, OPM will send notifications to approximately 4 million individuals whose Personally Identifiable Information (PII) may have been compromised," it said in a statement.
The OPM uncovered the hack in April.
It said it was offering credit monitoring and identity theft insurance for 18 months to those individuals potentially affected.
The FBI said it "will continue to investigate and hold accountable those who pose a threat in cyberspace."
It's the second time in a year that Chinese hackers were suspected of targeting the OPM. No personal information was reported stolen in the first instance.
NSA given wider online spy powers in search for hackers
Separately, it was reported on Thursday that the National Security Agency (NSA) was given wider powers to spy on Internet traffic in the search for computer hacking by foreign governments. The reports are based on documents leaked by former NSA contractor Edward Snowden.
The expanded authority was granted by the Justice Department back in 2012, according to the New York Times and ProPublica reports, and permits the NSA to sift through online data without a warrant.
It comes amid a renewed public debate in the US on whether the NSA's activities strike the right balance between privacy and security. Earlier this week, lawmakers in the Senate authorized a bill capping post-September 11 domestic surveillance by the NSA - reforming how the agency accesses and collects the phone records of Americans.
jr/bk (AFP, Reuters, AP)