South Korea's business sentiment weakened amid lingering concerns over external uncertainties such as Europe's debt crisis and high oil prices, the central bank said Tuesday.

The monthly business survey index (BSI), which gauges local manufacturers' assessment of current business conditions, was unchanged at 84 in May, but the seasonally-adjusted figure stayed at a lower level of 80 last month, according to the Bank of Korea (BOK).

The BSI for June, measuring manufacturers'outlook on business conditions for the upcoming month, retreated 4 points on-month to 86, and the reading adjusted with seasonal factors fell 2 points to 82, said the BOK.

The figure came amid a series of the country's economic outlook downgrade. The Organization for Economic Cooperation and Development (OECD) lowered its 2012 economic growth outlook for South Korea to 3.3 percent from an earlier estimate of 3.5 percent, citing uncertainties at home and abroad such as deterioration in the euro zone, rising oil prices and surging household debts.

Local manufacturers listed uncertain economic conditions, weak domestic demand and high oil prices as major difficulties in doing business.

Meanwhile, the monthly index for non-manufacturers fell 1 point on-month to 81 in May, and the index for June declined 2 points to 83. Local non-manufacturers regarded fragile domestic demand, fierce competition and economic uncertainties as their hardship.

The BSI index is based on a nationwide survey of 1,597 manufacturers and 872 non-manufacturers, conducted between May 15 and 22.