It is the biggest breach of government workers information in years.

Earlier on Thursday, the US Office of Personnel Management (OPM) announced that a cybersecurity breach possibly compromised the personal data of up to 4 million current and ex- US federal employees. David Cox Sr., the president of the American Federation of Government Employees, said he had been told that the breach might have affected ‘all 2.1 million current federal employees and an additional two million federal retirees and former employees.’…

But in Beijing Friday, the Chinese Foreign Ministry dismissed the allegations.

Foreign Ministry spokesman Hong Lei said on Friday: “Hacking attacks on the Internet are anonymous, global and hard to trace”. “It is irresponsible and unscientific to make conjectural, trumped-up allegations without deep investigation”.

Rob Pritchard is a cyber-security specialist at the Royal United Services Institute.

A U.S. official, who declined to be named because he was not authorized to publicly discuss the data breach, said the breach could potentially affect every federal agency.

However, OPM can be seen as a high-value hacking target.

Mr. Prichard said this information could be valuable to criminals who want to sell it for money, or state-sponsored hackers.

Li Haidong, a professor of U.S. studies at China Foreign Affairs University, said: “In U.S. political circles, the attitude toward China is at a turning point”.

One specialist said hackers could use information from government personnel files for financial gain.

“The FBI is conducting an investigation to identify how and why this occurred”, said the Homeland Security statement.

He said that rather than publicizing unproven allegations, Washington should have taken a more Chinese approach and quietly contacted Beijing to find a joint solution.

Her agency said the intrusion may have begun late a year ago and “predated the adoption of the tougher security controls”.

The agencies involved didn’t specify exactly what kind of information was accessed.

Meanwhile, the U.S. was trying to pick up the pieces.

Large American businesses have also been attacked.

China has always aggressively pushed back against United States hacking charges - describing them either as baseless, or simply hypocritical in the light of the NSA spying revelations leaked by Edward Snowden, the rogue intelligence contractor.

September 2014 - Senate investigators said that China’s military had hacked into computer networks of civilian transportation companies hired by the Pentagon at least nine times, breaking into computers aboard a commercial ship, targeting logistics companies and uploading malicious software onto an airline’s computers. “China has made great efforts to combat cyber attacks in accordance with Chinese laws and regulations”, he said. Hai Do was the editor.

VOA’s William Gallo wrote this story.