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.” The official said an emergency response team was assigned “to assess and mitigate any risks identified.”

One senior American official said that the attack was traced to China, though it was not clear if the hackers were part of the government. Its disclosure comes as a delegation of senior American 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.