Authorities are taking measures following the discovery of red fire ants at an apartment construction site in the southern city of Daegu.

The Environment Ministry said Tuesday seven ants were found with stones imported from China at the site, after a construction official alerted the Animal and Plant Quarantine Agency.

The seven ants were later confirmed to be worker ants not capable of reproduction.

The tiny pests, which are three to six millimeters long, are among the world's top 100 worst invasive species designated by the International Union for Conservation of Nature, as they are difficult to detect and are fast breeders that harm crops and disturb the ecosystem.

A red fire ant sting can cause severe pain, itchiness and even difficulty in breathing in worse cases.