WASHINGTON, Jan. 19 -- The U.S. National Security Agency (NSA) secretly broke into the computer systems of the Democratic People's Republic of Korea (DPRK) in 2010, the New York Times reported Monday, an apparent effort by the U.S. side to convince skeptics it has evidence that Pyongyang was behind last year's cyberattack on Sony Pictures Entertainment.

Citing former U.S. and foreign officials, computer experts briefed on the operations and a newly disclosed NSA document, the newspaper said the U.S. spy agency penetrated directly into the DPRK networks with the help of South Korea and other American allies after first hacking the Chinese networks that connect North Korea to the outside world.

The NSA began placing malware in the DPRK networks in 2010, first focusing on the DPRK's nuclear program and its leadership, but the focus shifted after a cyberattack in 2013 on South Korean banks and media companies, the report said.

As for the Sony attack, U.S. investigators concluded that the hackers spent more than two months, from mid-September to mid- November, mapping Sony's computer systems before carrying out the attack that began on Nov. 24.

The evidence gathered by the U.S. malware proved critical in persuading President Barack Obama to accuse the DPRK of ordering the Sony attack and to promise retaliation, which has begun in the form of new economic sanctions, it said.

It's the first time the United States has publicly charged another government with mounting a cyberattack on American targets.

But the New York Times also raised questions about why the United States was not able to alert Sony beforehand about the attack, which the U.S. said was probably caused by the release of "The Interview," a movie that features an assassination attempt against DPRK leader Kim Jong Un.

Pyongyang has repeatedly denied any involvement in the Sony hack.

Many experts are skeptical that DPRK was the culprit, or the lone culprit, suggesting that it was an insider, a disgruntled Sony ex-employee or an outside group mimicking DPRK hackers, the newspaper said.

"Many remain unconvinced," said the report. "It would not be that difficult for hackers who wanted to appear to be North Korean to fake their whereabouts."