WASHINGTON—U.S. authorities on Wednesday eyed North Korea as the origin of the widespread cyber attack that overwhelmed government Web sites in the United States and South Korea, although they warned it would be difficult to definitively identify the attackers quickly.

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.

The officials spoke on condition of anonymity because they were not authorized to speak publicly on the matter.

South Korea intelligence officials have identified North Korea as a suspect in those attacks and said that the sophistication of the assault suggested it was carried out at a higher level that just rogue or individual hackers.

U.S. officials would not go that far and declined to discuss publicly who may have instigated the intrusion or how it was done.

In an Associated Press interview, Philip Reitinger, deputy under secretary at the Homeland Security Department, said the far-reaching attacks demonstrate the importance of cybersecurity as a critical national security issue.

The fact that a series of computers were involved in an attack, Reitinger said, “doesn’t say anything about the ultimate source of the attack.”

“What it says is that those computers were as much a target of the attack as the eventual Web sites that are targets,” said Reitinger, who heads DHS cybersecurity operations. “They’re just zombies that are being used by some unseen third party to launch attacks against government and nongovernment Web sites.”

Targets of the most widespread cyber offensive of recent years also included the National Security Agency, Homeland Security Department and State Department, the Nasdaq stock market and The Washington Post, according to an early analysis of the software used in the attacks.

The Associated Press obtained the target list from security experts analyzing the attacks. They provided the list on condition of anonymity because they were not authorized to discuss the investigation.

Other experts in cyber assaults said the incident shined a harsh light on the U.S. government’s efforts to protect all of its agencies against Web-based attacks.

James Lewis, a senior fellow at the Center for Strategic and International Studies, said that the fact that both the White House and Defense Department were attacked but didn’t go down points to the need for coordinated government network defenses.

“It says that they were ready and the other guys weren’t ready,” he said. “We are disorganized. In the event of an attack, some places aren’t going to be able to defend themselves.”

The wave of cyber assaults are known as “denial of service” attacks. Such attacks against Web sites are not uncommon and are caused when sites are so deluged with Internet traffic that they are effectively taken off-line. Mounting such an attack can be relatively easy and inexpensive, using widely available hacking programs, and they become far more serious if hackers infect and tie thousands of computers together into “botnets.”