Cargo processed at South Korea's seaports slightly inched down from a year earlier in the first nine months due mostly to a drop domestic cargo, the government said Wednesday.

In the January-September period, the amount of cargo handled at the country's seaports came to some 998.6 million tons, down 0.3 percent from the same period in 2012, according to the Ministry of Oceans and Fisheries.

The slight drop was caused by a fall in the amount of domestic cargo, which dropped by nearly 2.6 percent on-year to about 169.2 million tons, the ministry said in a press release.

Import-export cargo, on the other hand, grew slightly from 827.8 million tons to 829.4 million tons over the cited period.

By port, the country's largest port in Busan processed some 241.9 million tons of cargo in the first nine months of the year, up 3.4 percent from the same period last year, while its second-largest port in southern Gwangyang handled 173.4 million tons of cargo, down 0.9 percent from a year earlier.

However, the amount of container cargo processed at South Korean seaports gained 3.3 percent on-year to some 17.4 million 20-foot equivalent units (TEUs) in the January-September period.

The ministry attributed the increase mostly to a 8.9 percent hike in transshipment cargo, or cargo processed here while en route to a final destination, often to change means of transportation.

Import-export container cargo also rose 1.4 percent on-year to 10.36 million TEUs over the cited period.