China on Friday rejected US accusation that the Chinese government is behind the breach of 4 million US federal employees' personal information, saying such accusations are "unscientific and irresponsible."

According to a CNN report, hackers working for the Chinese military are believed to have broken into the US government computer networks in one of the "biggest breach ever" and stolen the personal data of millions of former and current federal government employees.

"We've seen a lot of similar reports and accusations recently, but are they scientific?" Chinese foreign ministry spokesperson Hong Lei said at a regular press briefing on Friday.

Hong urged the US to stop being over-suspicious and pointing fingers based on shady evidence, and instead show more trust and cooperate with China in securing cyberspace security.

US officials said on Thursday that the Office of Personnel Management (OPM) suffered what appeared to be one of the largest breaches of information ever on government workers, Reuters reported.

The office handles employee records and security clearances.

A US law enforcement source told Reuters a "foreign entity or government" was believed to be behind the cyber attack. Authorities were looking into a possible Chinese connection, a source close to the matter said.

The FBI said it was investigating and aimed to bring to account those responsible.

Qin An, director of the China Institute of Cyberspace Strategy, told the Global Times Friday that the US government has been too eager to jump to conclusions by repeatedly pointing fingers at China.

Stealing personal information is low-level hacking and the US accusation that the Chinese government is behind the hack lacks evidence, said Qin.

The computer networks of all countries often have loopholes and are prone to cyber attacks. China's computer systems are also subject to hacking, such as the latest case involving the social security system, Qin said.

A local social security database was reportedly breached recently, which might have resulted in the leak of millions of people's personal data.

Qin said the US should not always blame China at a time when cyber attacks could be done by anybody and from anywhere. The two countries should join hands to face the challenges of Internet crimes.

The US government has several times accused the Chinese government of being behind hacker attacks on its networks, which China has repeatedly denied.

Several US states were already investigating a cyber attack on Anthem Inc, the second largest US health insurer, in February that a person familiar with the matter said is being examined for possible ties to China.

John Hultquist of Dallas-based iSight Partners told Reuters that the latest attack on OPM and the earlier breaches at Anthem appear to have been the work of hackers working on behalf of a state, not those focused on cyber crime.

He said they may have widened their net to gather personally identifiable information for more elaborate, finely-tuned attacks in the future. "This is usually done by criminals, but based on their behavior, we believe these are espionage actors," said Hultquist.

OPM detected new malicious activity affecting its information systems in April and the Department of Homeland Security said it concluded at the beginning of May that the agency's data had been compromised and about 4 million workers may have been affected.

The breach hit OPM's IT systems and its data stored at the Department of the Interior's data center, a shared service center for federal agencies, a DHS official said on condition of anonymity.

"The FBI is working with our inter-agency partners to investigate this matter," the bureau said in a statement. "We take all potential threats to public and private sector systems seriously, and will continue to investigate and hold accountable those who pose a threat in cyberspace."