South Korean President Moon Jae-in sacked his chief of staff and two other senior aides Tuesday in a major reshuffle seen as seeking to shore up his falling support.

But his numbers have plunged in recent weeks, falling below 50 per cent for the first time amid growing discontent over slow growth and unemployment, and disappointment over promised social reforms that critics say have not materialised.

Chief of staff Im Jong-seok announced his own dismissal to reporters at the presidential Blue House and replacement by Noh Young-min, the South's ambassador to China and a former three-term lawmaker.

Noh, 62, was a pro-democracy activist through the 1970s and 80s before entering politics in 2000 and eventually joining Moon's left-leaning Democratic Party.

Moon also replaced his senior political affairs secretary and senior press secretary in the biggest reshuffle at the Blue House since he took office in May 2017.

Moon has promised reforms of the powerful, family-controlled business groups that were embroiled in the Park scandal and to tackle widespread corruption.

But critics accuse him of being fixated on trying to build a political legacy by improving ties with the North while overlooking much-needed social reforms at home, and his flagship economic policy of "income-led growth" is accused of hurting those it is intended to help by raising employment costs.

Moon's ratings hit a low of 47 percent in December, according to the Seoul polling agency Realmeter.

The number inched up to 48.2 percent last week but remains far below the high 70s recorded in May after his first inter-Korean summit.

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