High levels of paralytic toxins have spread along South Korea's southeastern coast and contaminated shellfish, the maritime ministry said Wednesday, vowing to tighten oversight over the harvesting of the seafood.

Earlier this month, paralytic toxins were detected in waters near the southern port city of Busan and Geoje Island for the first time this year and have continued to spread to adjacent areas, according to the Ministry of Oceans and Fisheries.

A total of 28 regions are now confirmed to be contaminated, with paralytic toxin levels in "filter feeding" shellfish surpassing the allowed level of 80 micrograms per kilogram.

The hike is caused by microscopic algae and plankton, and warmer water and other environmental conditions appear to have caused a rise in toxin-producing algae, according to experts.

Shellfish including clams, mussels and oysters can retain the paralytic toxins as they eat the algae that produce harmful substances.

People and animals who consume tainted shellfish can suffer from diarrhea, abdominal pain, partial paralysis and, in serious cases, respiratory problems and even death.

Noting that the toxins cannot be destroyed by heating or freezing, the government has banned shellfish harvesting in the contaminated region while warning people against eating them.

"Two tests will be conducted per week to check plankton levels in uncontaminated areas of the oceans, and detailed information will be provided to the public to prevent possible accidents," ministry official Chung Bok-cheol said.

"We will further strengthen our monitoring system in cooperation with relevant agencies and regional governments," he added. (Yonhap)