WASHINGTON, June 5 (Reuters)rs included security clearance information and background checks dating back three decades, U.S. officials said on Friday, underlining the scope of one of the largest known cyber attacks on federal networks.

The breach of computer systems of the Office of Personnel Management was disclosed on Thursday by the Obama administration, which said records of up to 4 million current and former federal employees may have been compromised.

Accusations by U.S. government sources of a Chinese role in the cyber attack, including possible state sponsorship, could further strain ties between Washington and Beijing. Tensions are already heightened over Chinese assertiveness in pursuit of territorial claims in the South China Sea.

The hacking also raises questions about how the United States would respond if it confirmed that the Chinese government was behind it.

Several U.S. officials, who requested anonymity, said the hackers were believed to have been based in China but that it was not yet known if the Chinese government or criminal elements were involved.

Another U.S. official said the breach was being investigated as a matter of national security, meaning it may have originated from a foreign government.

The cyber attack was among the most extensive thefts of information on the federal work force, and one U.S. defense official said it was clearly aimed at gaining valuable information for intelligence purposes.

"This is deep. The data goes back to 1985," a U.S. official said. "This means that they potentially have information about retirees, and they could know what they did after leaving government."

Access to data from OPM's computers, such as birth dates, Social Security numbers and bank information, could help hackers test potential passwords to other sites, including those with information about weapons systems, the official said.

"That could give them a huge advantage," the official said.

According to a U.S. House of Representatives memo seen by Reuters, OPM knows what types of data were exposed to the hackers but not what data was taken. The memo was sent to House staff by Chief Administrative Officer Ed Cassidy, whose office provides support services to the House, including cyber security services.

In addition, the State Department said in a memo to its employees that most of them had not been exposed to the breach because their data was not housed on the hacked OPM systems. Only those who had previously been employed by another federal agency may have been exposed, it said.

Investigators have linked the OPM breach to earlier thefts of personal data from millions of records at Anthem Inc , the second largest U.S. health insurer, and Premera Blue Cross, a healthcare services provider.

It was the second computer break-in in less than a year at OPM, the federal government's personnel office, and the latest in a string of cyber attacks on U.S. agencies, some of which have been blamed on Chinese hackers.

A Chinese Foreign Ministry spokesman said such accusations had been frequent of late and were irresponsible. Hacking attacks were often cross-border and hard to trace, he said.

White House spokesman Josh Earnest said, "It's not clear who the perpetrators are," but he noted that President Barack Obama and his aides regularly raise with their Chinese counterparts concerns about Chinese behavior in cyberspace.

Disclosure of the latest computer breach comes ahead of the annual U.S.-China Strategic and Economic Dialogue scheduled for June 22-24 in Washington, D.C. Cyber security was already expected to be high on the agenda.

U.S. officials said the talks would proceed as scheduled, as would Obama's plans to host Chinese President Xi Jinping on a state visit to Washington in the fall.

U.S. LIKELY TO MOVE CAUTIOUSLY

At Friday's White House briefing, Earnest dodged the question of whether Washington might retaliate if it was determined that a state had been involved in the hacking.

In December, U.S. officials moved swiftly to accuse North Korea of being behind a high-profile attack on Sony over a movie depicting the assassination of North Korea's leader, and Obama vowed that the United States would respond.

Some lawmakers and defense officials want a more aggressive U.S. stance against cyber breaches, including legislation to strengthen U.S. cyber defenses. But the administration is likely to move cautiously in response to any Chinese role, mindful of the potential harm from escalating cyber warfare between the world's two biggest economies.

The Federal Bureau of Investigation has launched a probe of the OPM attack, and vowed that it would bring to account those responsible for the hacking.

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

Hackers 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.

Chinese hackers were also blamed for penetrating OPM's computer networks last year, The New York Times reported last July, citing unnamed U.S. officials.

James Lewis, a cyber security expert at the Center for Strategic and International Studies think tank, said the administration's disclosure of the hacking could be a signal to China of Washington's plan to push hard on cyber issues at this month's talks.

"The Chinese have been saying privately, and somewhat in public, that we want the summit to go really well. 'Let's not talk about espionage. Let's talk about how we can work together'," said Lewis, a former State Department official. "This might be a U.S. response to that: 'No, we are going to talk about espionage.'"