HONOLULU (AP) — The United States imposed fresh sanctions on North Korea on Friday, targeting top state officials and defense-related organizations in an attempt to punish North Korea for a crippling cyberattack against Sony. The sanctions marked the first public act of retribution by the U.S.

Although it was unclear how punishing the blow would be — North Korea already is under tough U.S. sanctions — the move signaled that that the U.S. was not backing away from its insistence that North Korea is responsible for the attack against Sony. North Korea has denied involvement, and some cybersecurity experts say it's possible Pyongyang wasn't to blame.

"The order is not targeted at the people of North Korea, but rather is aimed at the government of North Korea and its activities that threaten the United States and others," Obama wrote to a letter to House and Senate leaders.

The White House warned this was just the first part of the U.S. response to the Sony incident.

The stepped-up sanctions, authorized by President Barack Obama, will affect three North Korean entities that are already subject to some U.S. sanctions, plus another 10 individuals who work for those entities or the North Korean government. Although the U.S. already has sanctions in place against North Korea over its nuclear program, these are the first sanctions punishing Pyongyang for alleged cyberattacks.

The FBI has blamed North Korea for the crippling cyberattack against Sony Pictures Entertainment. North Korea has denied involvement but has expressed fury over a comedy film by Sony that mocked North Korea's leader. Sony Pictures initially called off release of the film, citing threats of terror attacks against U.S. movie theaters. Obama criticized Sony's decision, and the movie opened last month.

A nearly 10-hour shutdown of North Korean websites last week prompted widespread speculation that the U.S. had launched a counterattack in retribution, but the White House did not comment on whether the U.S. was responsible. The U.S. has vowed a proportional response to the Sony incident but has warned its response would "take place at a time and in a manner of our choosing."

North Korea and the U.S. remain technically in a state of war because the 1950-53 Korean War ended with an armistice, not a peace treaty. The rivals also are locked in an international standoff over North Korea's nuclear and missile programs and its alleged human rights abuses.

Among those sanctioned Friday are organizations tied closely to North Korea's defense industry:

—Korea Mining Development Trading Corporation, the state-owned arms dealer and exporter of equipment related to ballistic missiles and conventional weapons.

—Korea Tangun Trading Corporation, which obtains technology to support North Korea's defense research.

—Reconnaissance General Bureau, North Korea's primary intelligence organization that runs the country's cyber warfare.

Obama signed an executive order authorizing the sanctions from Hawaii, where he is on vacation with his family.

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Associated Press writer Nedra Pickler in Washington contributed to this report.

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Reach Josh Lederman on Twitter at http://twitter.com/joshledermanAP