North Korean hackers sent massive amounts of spam emails to South Korean public organizations last month, South Korea's police chief said Monday, the latest in a series of cyberattacks against the South in recent years.

"We are at a stage to be assured that it was committed by a North Korean hacking organization," said Kang Sin-myeong, commissioner-general of the National Police Agency, announcing the interim outcome of their probe into the case.

The hackers allegedly sent emails disguised as being sent by either the presidential office or the foreign ministry related to North Korea's recent nuclear test.

The Internet Protocol address -- the online equivalent of a street address or phone number -- used to send the spam emails was traced to China's northeastern province of Liaoning bordering North Korea. The network can be used wirelessly from North Korean territory, according to police.

It is the same Internet network behind a cyberattack on South Korea's nuclear power operator in 2014, according to Kang. North Korea is suspected of orchestrating the attack, though it has denied the charges.

Police said similar emails pretending to be from the presidential office or other government bodies have been sent from June 2015 to a total of 759 people.

After probing into 460 of the recipients' occupations, police said 87.8 percent have jobs related to North Korea.

"Looking at the probe results, there is a trace of an intentional and deliberate targeting process that cannot be deemed as a coincidence," Kang said.

Police also said some words in the mails are used only in North Korea.

Although South and North Koreans speak the same language, South Koreans do not understand some North Korean expressions.

The police chief said no major damage to national security has been confirmed due to the attack.

Police said they are planning to conduct an international joint investigation as they found two European servers used to send other spam mails disguised as a notice from a major South Korean portal site.

North Korea has a track record of waging cyber attacks on South Korea and the United States in recent years, though it has flatly denied any involvement.

The latest hacking attempt came amid heightened tensions on the Korean Peninsula after North Korea claimed in January that it successfully carried out its first hydrogen bomb test.

In response, South Korea is working with the U.S. and other regional powers to punish the communist country for its nuclear test and long-range missile launch that shortly followed the January provocation. (Yonhap)