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Pyongyang’s hackers now must be rated “as good as Iran,” said James A. Lewis, a cybersecurity scholar at the Washington-based Center for Strategic and International Studies.

“The Iranians moved up quickly,” Mr. Lewis said, noting the recent spate of “denial of service” attacks against U.S. banks laid at their door.

U.S. officials have said the greatest danger posed by cyberattacks is disruption of vital infrastructure, such as electric power transmission.

In some attacks on South Korean targets, the hackers had claimed to have stolen personal data as well, a move that Mr. Sherstobitoff described as “misdirection.”

“This was a psychological warfare move to make people think it was hacktivist, rather than nation-state,” he said.

The attackers also adopted the jargon of Anonymous and other hacktivists in the claims they posted online in an attempt to conceal their true nature, Mr. Sherstobitoff said.

McAfee researchers dubbed the attacks “Operation Troy” because of references to the classical city in the computer code the attackers used.