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U.S. and China Discuss Thorny Topic of Cybersecurity

As U.S. and Chinese officials meet in Washington, a massive data hack of millions of Americans’ personal information is the elephant in the room.

As U.S. and Chinese officials meet in Washington to discuss strategic and economic matters this week, the subject of a massive data hack of millions of Americans’ personal information is the elephant in the room.

On Tuesday, high-level officials, while carefully steering well clear of blaming China for what could be the biggest cyberattack in U.S. history, spoke frankly about the broader topic of “certain standards of behavior within cyberspace.”

Such actions, the officials said, could sour relations between the two nations.

The U.S. is “deeply concerned about government-sponsored cyber theft from companies and commercial sectors,” Treasury Secretary Jacob Lew said as the meetings got underway. He added that both nations have “responsibilities to abide by certain standards of behavior within cyberspace.”

Related: OPM Director on Second Hack: We Can't Say How Many Were Affected

It’s a sore subject, one the administration will continue to address tactfully but directly, a senior State Department Official told reporters during a background briefing on Monday.

“What you'll see is that in sort of working through these issues and making sure we have clear communication with the Chinese on our concerns about cybersecurity, we're able to raise it both with officials in the security side of the Chinese government, but also with officials in the economic side of the Chinese government,” the official said.

Earlier this month, the U.S. government announced that four million federal workers may have had their personal information compromised in the attack, which is thought to have originated in China and which officials said could affect every agency of the U.S. government.

The Obama administration has not attributed the hack to the Chinese government.

Some U.S. lawmakers initially identified the likely culprit as China, which has been suspected of involvement in previous government hacks.

Related: Officials: China Suspected of U.S. Data Breach Affecting Millions

Sen. Susan Collins, R-Maine, a member of the Senate Intelligence Committee, said the hack was "extremely sophisticated" and "that points to a nation state" as the responsible party, likely China. She later learned that she, too, is likely among the millions of federal government workers whose personal information was stolen in a cyberattack on the Office of Personnel Management, according to a letter obtained by NBC News.

After the hack was announced, China accused the United States of making "groundless accusations" and being "irresponsible" for trying to place blame with their nation.

China's foreign ministry spokesman Hong Lei told NBC News earlier this month that it was very hard to prove who was responsible for cyber attacks.

"Without the thorough investigation, you jump to a conclusion so quickly. We think it is not scientific and is irresponsible."

Last year, barely a month after the two countries agreed to work together on cybersecurity, China suspended cooperation with the U.S. on Monday after the Justice Department charged five Chinese military hackers with cyberespionage.

“It’s a delicate issue,” former CIA undercover officer-turned member of congress Rep. William Hurd, told NBC News.

All nations engage in espionage, however the U.S. government must do a better job of figuring out “what is a digital attack and what is the appropriate response,” said Hurd, a Texas Republican. He said he also received a letter that his personal information may have been stolen in the cyberattack.

On Wednesday, China’s State Councilor Yang Jiechi, speaking through a translator, said his country shares America’s concerns.

“We believe that cyber security is really important,” he said. “And we think nations should work together to develop a code of conduct.”