South Korea's opposition parties said on Friday they will vote on a motion to impeach scandal-tainted President Park Geun-hye on December 9, vowing to push ahead with a risky political move while her own party remains undecided whether to force her out.

Park has offered to resign and asked parliament on Tuesday to decide how and when she should step down over the influence peddling scandal but the opposition parties have rejected it as a ploy to buy time and avoid impeachment.

If Park is impeached or resigns she will be the first democratically elected South Korea president not to serve a full term.

The three opposition parties, with a combined 165 seats in the 300-member parliament, can bring the impeachment motion but will need some members from Park's Saenuri Party to bring the vote to the two-thirds majority required to pass the bill.

"The three opposition parties will pursue impeachment through close cooperation and without wavering," a spokesman for the main opposition Democratic Party, Ki Dong-min said.

"The motion will be proposed today. It will be reported to the plenary session on the 8th and we will bring the impeachment motion to a vote on the 9th," he said.

Park is accused of colluding with a personal friend, who has been indicted for abuse of power, to put undue pressure on the country's large conglomerates to contribute money to foundations that were set up to promote her policy initiatives. Park has denied any wrongdoings but has apologized to the nation.

Some members of Saenuri Party had earlier said they will join the opposition parties to impeach Park but have changed their position after Park offered to quit, saying she should be given the chance to step down on her own by April.

It was not clear whether there will be at least 28 members from Saenuri who will join the impeachment motion that are required to bring the vote to 200. There are seven non-party affiliated members who are expected to join the motion.

A failed impeachment motion can be immensely damaging to the opposition parties' push to force Park out of office.

If it passes, the Constitution Court has 180 days to approve or reject it. If approved, a new election must be called in 60 days to elect a new leader for a full five-year term.

Park has come under intense pressure to step down, with hundreds of thousands of people streaming into the streets demanding her resignation at weekend rallies.

Her approval rating remained at a record low of 4 percent, according to a Gallup Korea opinion poll released on Friday. Gallup Korea, based in Seoul, is not affiliated with US-based Gallup Inc.