WASHINGTON >> Chinese hackers in March broke into the computer networks of the U.S. government agency that houses the personal information of all federal employees, according to senior U.S. officials. They appeared to be targeting the files on tens of thousands of employees who have applied for top-secret security clearances.

The hackers gained access to some of the databases of the Office of Personnel Management before the federal authorities detected the threat and blocked them from the network, according to the officials. It is not yet clear how far the hackers penetrated the agency’s systems, in which applicants for security clearances list their foreign contacts, previous jobs and personal information, like past drug use.

In response to questions about the matter, a senior Department of Homeland Security official confirmed that the attack had occurred but said that, "at this time," neither the personnel agency nor Homeland Security had "identified any loss of personally identifiable information."

One senior U.S. official said that the attack was traced to China, although it was not clear if the hackers were part of the government. Its disclosure comes as a delegation of senior U.S. officials, led by Secretary of State John Kerry, are in Beijing for the annual Strategic and Economic Dialogue, the leading forum for discussion between the United States and China on their commercial relationships and their wary efforts to work together on economic and defense issues.

Computer intrusions have been a major source of disagreement between the countries, and the Chinese can point to evidence, revealed by Edward J. Snowden, that the National Security Agency went deep into the computer systems of Huawei, a major maker of computer network equipment, and ran many programs to intercept the conversations of Chinese leaders and the military.

U.S. officials say the attack on the Office of Personnel Management was notable because while hackers try to breach U.S. government servers nearly every day, they rarely succeed. One of the last attacks the government acknowledged occurred last year at the Department of Energy. In that case, hackers made off with employee and contractors’ personal data.

Just a month ago, the Justice Department indicted a group of Chinese hackers who work for the People’s Liberation Army Unit 61398 and charged them with stealing corporate secrets. But there is little chance that the Chinese military members will ever see the inside of an U.S. courtroom.