South Korea's parliamentary speaker Park Hee-tae on Thursday offered to resign amid allegations he tried to bribe his way to the chairmanship of the ruling party in 2008.

"I feel responsible for the problems involving myself and intend to resign," Park said in a statement read by parliamentary spokesman Han Jong-tae.

"Please let me be held responsible when others are found to have been involved" in the vote-buying scandal, Park added.

Park has been under pressure to resign after Koh Seung-duk of the ruling Saenuri Party, formerly the Grand National Party, said Park's aide offered him an envelope containing 3 million won (2, 688 U.S. dollars) in an attempt to buy his vote before the party leadership contest.

Park, who has denied his involvement in the alleged vote-buying scandal, was elected head of the conservative ruling party after the 2008 convention.

His two-year term was due to expire in May.

Liberal critics remain unmoved by his exit.

"The resignation offer came too late," Park Young-sun of the main opposition Democratic United Party said at a party meeting.

"Those also responsible should go ahead and turn themselves in, " party spokesperson Shin Kyung-min said, demanding a thorough investigation by prosecutors.

The scandal came as a blow to the conservative ruling party as it struggles to recover from a series of recent election defeats and other equally detrimental scandals ahead of key polls this year.

The parliament passed on Thursday a bill, allowing a special probe into an election-day hacking of the national election watchdog's website, which police said was orchestrated by two ruling party aides.

One of the aides, now arrested, was a secretary for Park.

The opposition camp has alleged high-ranking officials of the Saenuri Party might have ordered the attack to prevent younger, liberal voters from locating polling stations.

Park has denied his involvement in the cyber attack, but prosecutors are expected to soon summon him for questioning.