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SHANGHAI, Dec 27 (Reuters) - North Korea's Internet and 3G mobile networks were paralyzed again on Saturday evening, China's official Xinhua news agency reported on Saturday, with the North Korean government blaming the United States for systemic instability in the country's networks.

Internet connectivity had not returned to normal as of 21:30 local time, Xinhua reported, citing reporters in the country that had confirmed the situation over fixed telephone systems.

The report comes after the North Korean government called Obama a "monkey" and blamed the United States for enduring instability in the country's internet infrastructure, after the U.S. blamed North Korea for hacking attack on Sony Studios.

The attack was allegedly conducted to deter Sony from showing a comedy film called "The Interview," the plot of which featured a scheme to assassinate North Korean leader Kim Jong Un, and it resulted in major expense and embarrassment for Sony.

U.S. President Barack Obama promised retaliation for the attack but did not specify its form.

North Korea has denied responsibility for the attack on Sony.

Most of North Korea's heavily restricted internet traffic passes through Chinese routers on its way to the outside world, and some have suggested it is China - increasingly annoyed by recalcitrant North Korean behavior despite the two country's long-standing alliance - that is behind North Korea's internet troubles.

(Reporting by Pete Sweeney; Editing by Toby Chopra)