South Korea's key consumer sentiment index fell sharply in December from November on persistent concerns about the euro zone fiscal crisis and an already slowing global economy, a central bank survey showed on Tuesday.

The Bank of Korea said in a statement its consumer sentiment index fell to 99 in December from 103 in November, dropping below the 100-point mark for the third time in five months.

A reading below 100 means consumers expecting future economic and living conditions to deteriorate outnumber those predicting improvement. The four-point fall in December marked the biggest monthly loss since March this year.