The United States' intelligence chief said on Thursday that China was the top suspect in a hack of a U.S. agency that compromised the personnel records of millions of Americans, the Wall Street Journal reported on Thursday.

The comments from Director of National Intelligence (DNI) James Clapper marked a departure of sorts for the Obama administration, which has avoided publicly pointing to Beijing, even as officials said privately China likely was behind the attack.

"You have to kind of salute the Chinese for what they did," given the difficulty of the intrusion, the Journal quoted Clapper as saying at a Washington intelligence conference.

In a statement, Clapper's office said: "The DNI was clear China remains a leading suspect, though the USG continues to investigate," using an acronym for the U.S. Government. It referred any other questions on the investigation to the FBI.

The Office of Personnel Management said this month that personnel data on 4.2 million current and former federal employees had been compromised in the attack, although some media have reported that as many as 18 million Americans could have been affected.

White House spokesman Josh Earnest on Thursday declined to comment on any potential suspects for the attack.

He cautioned against guessing at what response the United States might take against those responsible. "If there is a response, it's probably not one we are likely to telegraph in advance," Earnest said.

The Journal cited Clapper as saying the U.S. government and American companies would continue to be targets until policymakers addressed the "lack of deterrents."

He said the absence of a U.S. threat to respond to hacking attacks meant Washington had to put its focus instead on defense, the newspaper reported.

U.S. officials speaking on background have said they suspect China was behind that hack, but the Obama administration has not publicly accused Beijing or identified the attackers. China has denied any involvement in the breach.