SEOUL • South Korea's spy agency has admitted it had engaged in a far-reaching attempt to manipulate voters as it sought to help conservatives win parliamentary and presidential elections.

In-house investigators from the National Intelligence Service (NIS) confirmed that the agency's cyber warfare unit organised and operated up to 30 teams for more than two years in the run-up to the 2012 elections, the agency said in a statement late on Thursday.

They hired Internet-savvy civilians and sought to sway voter opinions through postings on portals and Twitter.

Former NIS chief Won Sei Hoon, who led the agency from 2009 to 2013, was found to have ordered the operation.

"The teams were charged with spreading pro-government opinions and suppressing anti-government views, branding them as pro-North Korean forces' attempts to disturb state affairs," it said.

At the time, the country was led by the conservative Lee Myung Bak. The December 2012 presidential election was won by his now-disgraced colleague Park Geun Hye, who defeated liberal Moon Jae In.

Mr Moon won South Korea's presidential vote in May this year after Park was impeached and dismissed over corruption and abuse of power, and ordered a probe.

He has vowed to reform the NIS to prevent it from meddling in elections, and make it focus on collecting and analysing intelligence on North Korea and foreign affairs.

Mr Moon's Democratic Party yesterday demanded a probe into former president Lee over the spy agency's smear campaign in the 2012 presidential election.

A spokesman for Park's party, now in opposition and renamed Liberty Korea, said the NIS inquiry was "politically motivated". "The NIS says it will dissociate itself from politics but it is meddling in politics again by starting this probe," Mr Kang Hyo Sang said.

Former spy chief Won is being tried again for leading an online smear campaign against Mr Moon, after his initial conviction was overturned on appeal.

But the NIS investigation results suggest the scale of the voter manipulation was far wider than previously thought.

The internal probe also found Won ordered the agency to muzzle the press, provide support for pro-government conservative civic groups and put some major opposition politicians under secret surveillance.

The Democratic Party said those involved in the smear campaign should be punished as a warning to others, calling on former president Lee to confess to the truth and take responsibility for any wrongdoing.

The NIS has been tainted by a series of scandals, including the forging of documents to build a false spying case against a former Seoul city official who had escaped to South Korea from the North in 2004.

AGENCE FRANCE-PRESSE, XINHUA