(Bloomberg) -- North Korea’s Internet has been hit with outages and is offline today, according to a network-monitoring company, days after the U.S. government accused the country of hacking into Sony Corp.’s files.

North Korea, which has four official networks connecting the country to the Internet -- all of which route through China -- began experiencing intermittent problems yesterday and today went completely black, according to Doug Madory, director of Internet analysis at Dyn Research in Hanover, New Hampshire.

U.S. President Barack Obama said last week that Sony Pictures Entertainment had suffered significant damage and vowed to respond. North Korea warned yesterday that any U.S. punishment over the hacking attack on would lead to a retaliation “thousands of times greater.” North Korea has said it doesn’t know the identity of the hackers -- who call themselves “‘Guardians of Peace’’ -- claiming responsibility for breaking into Sony’s computer network and divulging internal e-mail messages.

‘‘The situation now is they are totally offline,’’ Madory said. ‘‘I don’t know that someone is launching a cyber-attack against North Korea, but this isn’t normal for them. Usually they are up solid. It is kind of out of the ordinary. This is not like anything I’ve seen before.”

While North Korea has four networks connected to the Internet, the U.S. has more than 152,000 such networks, according to the researcher.

The outage probably isn’t a cut of a fiber-optic cable, which would be shown in an immediate loss of connectivity, and other possible explanations include a software meltdown on North Korea’s Web routers or denial-of-service hacking attacks, Madory said.