Outbound travelers line up at ticket couters at Incheon International Airport, Friday. Yonhap

Summer vacationers who bring back nutritional supplements and seasoning products after traveling abroad should make sure that those items do not include substances that are banned in Korea, the nation's customs agency said Monday.

The Korea Customs Service (KCS) made the announcement amid a surge in pent-up demand for overseas travel and an increased risk of travelers returning with prohibited items, whether intentionally or unintentionally.

In response, the KCS revealed that it will initiate a special campaign from July 22 to Aug. 11 to promote more honest customs declarations among travelers.

"We've had cases of travelers who possess nutritional supplements and seasonings with addictive substances," the customs agency said. "These substances are permitted for medical and culinary purposes in other countries but are strictly prohibited in Korea."

Bagel seasoning products like this one are popular among Korean tourists to the U.S. Screenshot from Trader Joe's website

For example, seasoning products that are sold at various stores in the United States have poppy seeds, which may contain trace amounts of opiate residue.

"Bringing in those items is against the law and we ask travelers to be extra careful when purchasing the items," it said.

Concerning customs declarations, the KCS stressed that the duty-free allowance for inbound travelers is $800 per person.

For duty-free items, the KCS specifies that travelers can bring in one 1-liter bottle of alcohol worth up to $400 and 100 packs of cigarettes.

Those who do not make an honest declaration and are caught will face an additional tax penalty amounting to 40% of the tax they owe.