Suspected Chinese hackers who infiltrated the US government's human resources records have sensitive information on all federal employees, an American union said Thursday.

In a letter to authorities, the American Federation of Government Employees said it believes hackers -- who others have linked to China -- obtained confidential identification numbers that each American needs to get a job, a driving license or a bank account.

"We believe that Social Security numbers were not encrypted, a cybersecurity failure that is absolutely indefensible and outrageous," union president David Cox said in a letter to the government's Office of Personnel Management, subject of the vast breach.

"The hackers are now in possession of all personnel data for every federal employee, every federal retiree, and up to one million former federal employees."

The union said that data concerning an individual's address, date of birth, salary and military record were also compromised.

The US government last week admitted that data linked to at least four million current and former federal employees was hacked.

The hack of the Office of Personnel Management included records on 750,000 Department of Defense civilian personnel.

"The precise scope of this particular intrusion is one that continues to be under investigation by the FBI and other technical experts," White House press secretary Josh Earnest said Thursday.

"We have already begun the process of contacting those that we, thus far, believe could potentially have been affected."

China operates a vast domestic security and surveillance apparatus, but Beijing last week labelled as "irresponsible" US media reports that said Chinese hackers were behind the massive cyberattack.

Zhu Haiquan, spokesman for the Chinese embassy in Washington, said Chinese laws prohibit cybercrimes of all forms and said Beijing was trying to combat cyberattacks "in accordance with Chinese laws and regulations."

But The New York Times this week reported that the hackers in the Office of Personnel Management intrusion may have snared the names of Chinese with links to American officials, potentially putting them in danger.

Investigators say hackers could have obtained the names of Chinese relatives, friends and associates of American diplomats and other government officials, the Times said, leaving those Chinese open to blackmail or retaliation.

The breach at the Office of Personnel Management was just the latest in a series of major incursions that have shown the vulnerability of the US federal government, experts say.

In 2013, the US Internet security firm Mandiant said its hundreds of investigations showed that groups hacking into US newspapers, government agencies and companies "are based primarily in China and that the Chinese government is aware of them."