WASHINGTON (AFP) - North Korea appeared to have been hit by Internet outages, reports said on Monday, just days after US President Barack Obama warned that Washington would retaliate for a cyber attack on Sony Pictures blamed on Pyongyang.

The New York Times reported that the country's Internet had gone "completely dark".

According to respected US-based cybersecurity firm Dyn Research, Internet connectivity between the remote North Korea and the outside world, never good at the best of times, seemed to have been severely affected in recent hours - suggesting it may be under attack.

"I haven't seen such a steady beat of routing instability and outages in KP (North Korea) before," said Mr Doug Madory, director of Internet analysis at Dyn Research.

"Usually there are isolated blips, not continuous connectivity problems. I wouldn't be surprised if they are absorbing some sort of attack presently."

Washington accuses Pyongyang of being behind the hack that led to the release of embarrassing company emails and caused Sony executives to halt the debut of the madcap movie The Interview. The film about a fictional CIA plot to kill the country's leader infuriated North Korea, although Pyongyang has repeatedly denied it was behind the cyber assault.

Mr Obama, while saying that the alleged hack was not an act of war, has promised an unspecified "proportionate" response.

Deputy State Department spokeswoman Marie Harf said she could not comment on the online reports of Pyongyang's Internet woes.

The US administration is "discussing a range of options" in response to the Sony hacking, she said.

"As we implement our responses, some will be seen, some may not be seen," she said, stressing Washington would not publicly outline its moves.