South Korea's defense ministry said Tuesday it has launched an investigation into allegations that its Cyber Warfare Command posted online messages in favor of the ruling camp during last year's presidential campaign, shedding light on the secretive unit.

The ministry established the 400-member special unit under its wing in 2010 to expand its warfare capability on cyber space and counter threats from North Korea, which is believed to have trained professional hackers and attacked the South Korean government's and companies' websites.

The left-wing newspaper Hankyoreh on Tuesday reported that soldiers and three personnel at the command posted about 300 online messages against then-opposition camp candidate Moon Jae-in ahead of the presidential election held last December.

The latest allegation comes at a politically sensitive time as the National Intelligence Service (NIS) has been under attack for launching an online smear campaign to sway public opinion in favor of President Park Geun-hye, the then ruling party candidate, ahead of the election.

Former NIS chief Won Sei-hoon, currently under arrest, is going through court trials on charges of illegal electioneering. 

The defense ministry flatly denied any involvement in the election meddling activities, saying it has launched a special team to investigate the allegations.

"The defense ministry ordered soldiers to maintain political neutrality five times during the election period," ministry spokesman Kim Min-seok said in a briefing. "The Cyber Command also stressed four times that its members should stay politically neutral."

The ministry will announce its investigation results to clear out the allegations and punish those responsible if any of them are found to have violated regulations, he said.

Kim, however, remained cautious over details of the special unit kept largely secret, saying, "There is no country in the world that unveils its military operational plans."

On Monday, Rep. Kim Kwang-jin of the Democratic Party (DP) raised the allegations that members of a psychological warfare command, called the "530 unit" were rewarded for launching online campaigns in favor of the ruling camp.

"It is known that the Cyber Warfare Command's 530 unit members posted writings during the presidential election period," Kim said.

"The cyber psychological warfare unit members were awarded shortly after the presidential election. What were they rewarded for?"

It was the first time that a specific unit's name was mentioned in the public space as details of its organization are classified as confidential military information.

Defense Minister Kim Kwan-jin said the members were not involved in the election, saying their activities were "against North Korea, which denies the existence of the Republic of Korea through their propaganda."

The North is known to be running a cyber warfare unit, called "No. 121," in which about 3,000 elite hackers break into other computer networks for information and spread computer viruses. (Yonhap News)