President Barack Obama vowed Friday to punish North Korea for hacking a Hollywood studio, but Washington's options are limited and Pyongyang's economic weakness is a surprising strength.

No one expects the United States to launch a military strike against a nuclear-armed provocateur, but sanctions against its tiny economy or cyber attacks on its ramshackle Internet would achieve little.

"I'm sure they're exploring covert options, but also looking at it through the prism of -- 'we don't want to start an armed conflict on the Korean peninsula'," said cyber war expert James Lewis.

According to the US government, Kim Jong-Un's secretive state was behind a cyber attack that humiliated major studio Sony Pictures and forced it to cancel a movie mocking him.

The hack outraged US lawmakers -- foreign policy hawk Senator John McCain called it an "act of war" -- and triggered calls for North Korea to face sanctions and be re-designated a state sponsor of terror.

Former president George W. Bush removed North Korea from the list of terror states in 2008 in the hope of encouraging talks to halt its ongoing bid to deploy nuclear-armed missiles.

But Pyongyang has remained a wildly unpredictable and reckless foe -- and is still technically at war with the United States given the absence of a treaty to end their 1950-1953 conflict.

Lewis, a fellow at the Center for Strategic and International Studies think tank and a former government adviser, said that given the risk of escalation, the US response would be limited -- and thus probably largely ineffective.

"The North Koreans get a kind of impunity because of their unique status," he said, noting that sanctions would change little.

"North Korea has another advantage -- they don't have an economy."

- Cyber war 'cheap' for N.Korea -

US officials have outlined a doctrine for responding to cyber attacks on critical government, industrial or financial infrastructure -- but it is not clear if a movie studio meets this definition.

The Federal Bureau of Investigation said the Sony hack had been traced to North Korea in part through its similarities to attacks launched last year against banks and media outlets in South Korea.

The malicious code that scoured Sony's network was a customized version of the known hacking tool "Destover," which was used in attacks on South Korean banks already pinned on the North.

"North Korea has been in everyone's sights for years for trying to develop cyber war capabilities," said Tim Stevens of the War Studies department at King's College London.

"It's a relatively cheap option. There are reports that they are running a hacking unit out of China with at least the tacit consent of the PLA," he said, referring to China's People's Liberation Army.

Obama said that the US has no evidence that North Korea received assistance from another country in mounting the attack, but its own Internet capabilities are not known to be fearsome.

"The interesting thing is that, if North Korea develops a strong capability, it will be much easier for them to carry out attacks than for other countries to retaliate," Stevens said.

"Their Internet infrastructure is so rudimentary that it would be hard to cause many problems for North Korea. So it's a strategic win-win for them," he told AFP.

Lewis agreed that the response this time will likely have little impact, but would be worthwhile at least in showing other countries that the United States can track cyber attacks and is ready to act.

- Elite cyber squad -

Many experts questioned the true extent of the North's skills, but Yang Moo-Jin of Seoul's University of North Korean Studies said it had a track record in the style of assault directed at Sony.

"Its cyber-attacking abilities are good enough to hack into government and private websites in South Korea, and other countries in general," he said.

According to Yang, North Korea has in recent years trained thousands of hackers in secret government and military programs.

South Korean intelligence believes North Korea runs an elite cyber-warfare unit with at least 3,000 personnel, and regards its ability to launch hacking attacks to be a major security threat.

In recent years, hackers have deployed malware and virus-carrying emails against South Korean military institutions, commercial banks, government agencies, TV broadcasters and media websites.

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