WASHINGTON — The U.S. government was not responsible for sustained electronic attacks that crippled North Korea’s Internet infrastructure last month, just after President Barack Obama promised that his administration would respond to the hacker break-in at Sony Pictures Entertainment, two senior U.S. officials told The Associated Press.

The Obama administration has been deliberately coy about whether it caused North Korea’s outage, which affected all the nation’s Internet connections starting the weekend of Dec. 20. But the two officials, speaking on condition of anonymity, acknowledged to the AP it was not a U.S. operation.

It was not immediately clear even within the administration whether rogue hackers or other governments disrupted North Korea’s networks. The networks are not considered especially robust since they rely on a single provider, China United Network Communications Group, the state-owned provider in neighboring China. North Korea’s service was sporadic starting Saturday, Dec. 20, then collapsed entirely for nearly 10 hours two days later.

“It looks more like the result of an infrastructure attack than an infrastructure failure,” said James Cowie, chief scientist at Dynamic Network Services of Manchester, N.H.