WASHINGTON (AFP) - North Korea's connections to the Internet were cut for the second day running Tuesday, according to a US Internet research group that has been tracking the country's struggle to stay online.

Dyn Research said North Korea's four online networks, all supplied by Chinese communications firm China Unicom, went offline at 1541 GMT (11:41 pm Singapore time Tuesday) after becoming unstable over the preceding hours.

On Monday, North Korea had been cut off from the Internet for more than nine hours, triggering speculation that the isolated dictatorship had been targeted by a US cyber attack.

Washington has accused North Korea of being behind the hacking of Hollywood studio Sony Pictures, and President Barack Obama has promised a "proportionate response" to the alleged attack.

The cause of the outages in North Korea's already limited Internet access has not been confirmed, but experts said that the kind of crash it suffered resembles that caused by a "denial of service" attack.