SEOUL, South Korea — Key North Korean websites were back online today after a nearly 10-hour shutdown that followed a U.S. vow to respond to a crippling cyberattack on Sony Pictures that Washington blames on Pyongyang.

It wasn’t immediately clear what caused the Internet stoppage in one of the least-wired and poorest countries in the world, but outside experts said it could be anything from a cyberattack to a simple power failure. The White House and the State Department declined to say whether the U.S. government was responsible.

Even if a cyberattack had caused the shutdown, analysts said, it would largely be symbolic since only a tiny number of North Koreans are allowed on the Internet — a fraction of Pyongyang’s staunchly loyal elite, as well as foreigners.

Though it denies responsibility for the Sony hack, North Korea’s government has called it a “righteous deed” and made clear its fury over Sony’s film “The Interview,” a comedy that depicts the assassination of the North’s authoritarian leader, Kim Jong Un, the head of a 1.2 million-man army and the focus of an intense cult of personality.

South Korean officials, speaking on condition of anonymity because of office rules, said the North’s official Korean Central News Agency and the Rodong Sinmun newspaper, which are the main channels for official North Korean news, had both gone down. But the websites were back up later today. Among the posts glorifying the ruling Kim family included one about Kim Jong Un visiting a catfish farm.

U.S. computer experts described the Internet outage in the North as sweeping and progressively worse. Jim Cowie, chief scientist at Dyn Research, an Internet performance company, said in an online post that the North came back online after a 9½-hour outage.

Possible causes for the shutdown include an external attack on its fragile network or even just power problems, Cowie wrote. But, he added, “We can only guess.”

The outage was probably more inconvenient to foreigners than to North Koreans, most of whom have never gone online. There are only about 1,000 Internet Protocol addresses in North Korea for a population of 25 million, South Korean analysts say. The privileged are also allowed to view a self-contained domestic intranet that carries state media propaganda and a limited amount of information pulled and censored from the Internet.