Local media outlets have reported on the allegations involving Woo Byung-woo, the top presidential secretary for civil affairs.

Woo called the allegations "utterly groundless" and filed complaints with the prosecution against two newspapers that raised the claims.

"Amplifying the allegations without legitimate evidence is an irresponsible act and would not be helpful in forging national unity," a senior official at Cheong Wa Dae told reporters.

"I believe they should refrain from waging any unilateral political offensive or making any move to shake the government's management of state affairs, at a time when the president and the government are making all-out efforts to tide over security and economic challenges."

Reports said that Woo served as an attorney to secretly defend Jung Woon-ho, former chief of local cosmetics brand Nature Republic. The disgraced businessman has been detained in a lobbying scandal that allegedly ensnared preeminent figures in the judiciary.

In a press release, Woo said that he had never met Jung before.

He called the reports "completely false and fictitious."

Woo, a former prosecutor, briefly worked as a lawyer between 2013 and 2014.

A day earlier, another report said that in 2011, Korea's leading online game maker Nexon Co. purchased an expensive building in the posh southern district of Seoul, which was owned by the family of Woo's wife.

The report raised the suspicion that Jin Kyung-joon, a senior prosecutor who was arrested in a bribery case, might have played a role as a broker in Nexon's purchase of the building worth 130 billion won ($113 million).

Jin, known to be close to Woo, is Nexon's founder Kim Jung-ju's friend from college.

Kim is currently under investigation over his alleged involvement in Jin's stock transactions. Nexon allegedly lent some 400 million won to Jin in 2005, helping the prosecutor amass some 12 billion won in profits through stock transactions by 2015.

Woo claims that his wife's family sold the building to Nexon through a "normal" transaction process.

The prosecution in Seoul initiated an investigation into the case after Woo filed a complaint against the local daily Chosun Ilbo that first published an article on the case on Monday.

With regard to the case, a civic group also filed a complaint against Woo, alleging that the presidential official might have helped Jin get promoted in return for Jin's help in selling the building.

Opposition parties called on President Park Geun-hye to dismiss Woo and launch a probe into the allegations.

"This issue is a very serious one given that it concerns a powerful figure of the government," Rep. Woo Sang-ho, floor leader of the main opposition Minjoo Party of Korea, said during a meeting with party officials at the National Assembly.

"The president should unveil her position on how to conduct a probe (into the allegations)." (Yonhap)