The powerful attack that targeted dozens of government and private sites underscored how unevenly prepared the U.S. government is to block such multipronged assaults.

While Treasury Department and Federal Trade Commission Web sites were shut down by the software attack, which lasted for days over the holiday weekend, others such as the Pentagon and the White House were able to fend it off with little disruption.

The North Korea link, described by three officials, more firmly connected the U.S. attacks to another wave of cyber assaults that hit government agencies Tuesday in South Korea. The officials said that while Internet addresses have been traced to North Korea, that does not necessarily mean the attack involved the Pyongyang government.