Thailand was the biggest single source of smuggled illicit drugs seized in South Korea last year.
The kingdom accounted for 187 kilogrammes of the total of 780kg, or 24%, confiscated by South Korean authorities last year, Yonhap news agency reported on Tuesday, citing government data.
The United States was second with 20%, followed by Germany, 12%, and Laos, 9%, the report said.
South Korea confiscated 184kg of illicit drugs during the first four months of this year, a 13% decrease from the same period last year, but the number of attempts rose 14% to 234, according to the Korea Customs Service (KCS).
South Korea will set up a new customs inspection section at Incheon International Airport, the gateway to the country, by the end of next month to screen all carry-ons and check passengers arriving at the airport from countries producing or transiting illicit drugs.
"We've tightened border control, which has led to a recent fall in large-scale smuggling attempts. Our focus will also be on detecting small-scale smuggling cases," Yonhap quoted a KCS agency official as saying.
Last month in Nong Khai, police arrested a 70-year-old man and seized 22 kilogrammes of heroin reportedly bound for South Korea.