South Korean prosecutors on Monday questioned two former key presidential aides over allegations they helped a shadowy confidant of President Park Geun-Hye meddle in state affairs and secretly visit her office.

Park has been engulfed by a scandal that centres on Choi Soon-Sil, who is accused of using her personal ties with the president to coerce local firms to donate millions of dollars to non-profit foundations Choi then used for personal gain.

Choi, 60, is also accused of interfering in state affairs to the extent of nominating officials and editing Park's speeches even though she has no official title or security clearance.

Ahn Bong-Geun and Lee Jae-Man, who served as Park's key advisors until last month, are accused of helping Choi to visit the presidential office.

They also allegedly reported state affairs or leaked confidential documents to her.

TV news footage showed the pair separately entering the Seoul prosecutors' office on Monday morning as they were mobbed by reporters.

The two, who served as Park's aides for decades, were described by local media as ‘doorknobs’ to the president who wielded enormous power over policymaking.

Two other presidential aides have been arrested in the snowballing influence-peddling scandal.

- Independent probe -

Ruling and opposition lawmakers, meanwhile, agreed Monday to appoint an independent team of prosecutors to probe the widening scandal.

Under the agreement, a special prosecutor will be recommended by the two opposition parties, a spokeswoman for the main opposition Democratic Party of Korea told AFP.

The deal comes after opposition parties said they were not satisfied with the existing investigation.

The new, larger team -- consisting of more than 60 prosecutors and other investigators -- will be given 120 days to look into allegations against Choi and a handful of presidential aides mired in the scandal.

The spokeswoman said the agreement would be passed at a plenary session scheduled for Thursday.

Choi, whose father was an elusive religious figure and a longtime mentor to Park until his death in 1994, was arrested earlier this month for abuse of power and fraud.

The scandal has sparked nationwide fury with hundreds of thousands of people taking to the streets to call for Park's resignation and mocking the president as Choi's ‘puppet’.

A mass rally held in Seoul on Saturday drew one million people, according to organisers, making it the largest public protest in the South for nearly three decades.

Park also faces allegations that she helped Choi extract money from local companies including Samsung and Hyundai. Prosecutors are seeking to formally question her this week.

If she agrees, Park will become the first South Korean president to be probed while in office.

Under South Korea's constitution, the incumbent president may not be charged with a criminal offence except insurrection or treason.

But many argue the sitting president can be investigated by prosecutors and then charged after leaving office.