South Korean President Yoon Suk Yeol promised "reform" and the head of his ruling party resigned on Thursday after a disastrous election increased the opposition's stranglehold on parliament.

In addition to People Power Party (PPP) leader Han Dong-hoon, Prime Minister Han Duck-soo and a string of senior aides also offered to step down, local media said.

The result sets the stage for a legislative stalemate over President Yoon's remaining three years in office, just as the country faces challenges including a weak economy and an increasingly aggressive North Korea.

"I will humbly honour the will of the people expressed in the general election, reform the state affairs, and do my best to stabilise the economy and people's livelihood," Yoon said, according to his chief of staff Lee Kwan-sup.

Near final results from Wednesday's vote showed Yoon's conservative PPP and its satellite sliding from 114 seats in parliament to just 108, according to National Election Commission figures.

The big winners were Lee Jae-myung's Democratic Party (DP) and its partner, which saw their seat tally rise to 174 from 156 in the outgoing legislature.

The newly-founded Rebuilding Korea party, led by former justice minister Cho Kuk, capitalised on discontent with the two main parties to pick up 12 seats.

The landslide was however less emphatic than suggested by exit polls, with all opposition parties combined falling short of a super-majority of 200 seats in the 300-strong National Assembly.

South Korea uses a mix of first-past-the-post seats and proportional representation and has one legislative chamber. Presidents can serve only one five-year term. (AFP)