White House officials said that no classified networks had been compromised, and that the hackers had collected no classified information. Many senior officials have two computers in their offices, one operating on a highly secure classified network and another connected to the outside world for unclassified communications.

But officials have conceded that the unclassified system routinely contains much information that is considered highly sensitive: schedules, email exchanges with ambassadors and diplomats, discussions of pending personnel moves and legislation, and, inevitably, some debate about policy.

Officials did not disclose the number of Mr. Obama’s emails that were harvested by hackers, nor the sensitivity of their content. The president’s email account itself does not appear to have been hacked. Aides say that most of Mr. Obama’s classified briefings — such as the morning Presidential Daily Brief — are delivered orally or on paper (sometimes supplemented by an iPad system connected to classified networks) and that they are usually confined to the Oval Office or the Situation Room.