South Korean manufacturers saw revenue fall for two straight years in 2015, indicating a slowdown of the economy's main growth engine, central bank data showed on Friday.

Revenue of South Korean companies subject to external audit reduced 2.4 percent in 2015 from a year earlier, according to the Bank of Korea (BOK). It was faster decline than a 0.3 percent fall in 2014.

It was based on an assessment of 19,367 companies with an asset of more than 12 billion won (10.2 million U.S. dollars), including 9,506 manufacturers and 9,861 non-manufacturers.

Revenue among manufacturers declined 4.2 percent last year on a yearly basis after falling 1.9 percent in 2014. It was a downturn from a 1.2 percent increase in 2013.

The reduction for two straight years was attributable to the global economic slowdown and cheaper crude oil that led to flagging exports and damaged local exporters.

The sluggish manufacturing industry boosted worries as manufacturing exporters has led the country's economic growth.

Revenue among non-manufacturers, including those in the fishery, mining, electricity, gas, construction and service sectors, inched up 0.1 percent in 2015, down from rises of 2.2 percent in 2014 and 3.1 percent in 2013 each.

Despite the falling revenue, profitability improved last year thanks to lower energy costs caused by low prices of raw materials.

The ratio of operating profit to revenue was 5.2 percent in 2015, up 0.9 percentage points from a year earlier.

The rate among manufacturers increased from 4.4 percent in 2014 to 5.4 percent in 2015, with the figure for non-manufacturers gaining from 4.2 percent to 4.9 percent.