A prominent South Korea opposition lawmaker was found dead Monday, apparently after jumping from an apartment building amid an investigation into allegations that he accepted illegal political funds, Yonhap news agency reported.

Police officials said the body of Rep Roh Hoe-chan, floor leader of the progressive Justice Party, was found near the entrance of the apartment building in central Seoul at 9.38am local time.

A security guard alerted police of the finding, officials said.

Police then found a jacket on the stairs of the building's 17th and 18th floors. In the jacket were Roh's wallet containing his ID and business cards, as well as what appeared to be a suicide note, officials said.

In the note, Roh said he accepted money, but it had nothing to do with illicit favours. Roh also apologised to his family.

"I heard a bang and went there to find Rep Roh," said the security guard surnamed Kim. "I immediately reported the incident to the police without touching anything. After the police told me to feel his pulse, I tried to do so, but there was no pulse even though it was just one or two minutes after his fall."

Police said Roh's mother and brother live in the apartment building.

The three-term lawmaker has been under an investigation into suspicions that he accepted 50 million won (US$44,333) from an aide to a power blogger, known by the nickname Druking, at the centre of an opinion rigging scandal.

Roh, a former labour activist who built an image as a clean politician, has denied the allegations.

The case was investigated in 2016, but no charges were filed at the time.

The team of special prosecutor Huh Ik-bum began looking into the case again as part of the investigation of Druking and found that it was because Druking's aide submitted fabricated evidence to investigators that no charges were filed at the time.

Independent counsel Huh expressed sorrow over the death.

"It's truly heartbreaking to hear the unexpected sad news," Huh, wearing a black tie, told reporters. "I pray for his soul and offer words of consolation to his family."

Huh also bowed his head deeply before cameras.

The presidential office Cheong Wa Dae also mourned Roh's death.

"There was a heartbreaking incident this morning," presidential spokesman Kim Eui-Kyeom told reporters. "I hope Rep Roh will rest in peace."

The office also called off an Internet broadcasting program in which Moon was scheduled to appear and give answers to online petitions.

--Bernama