Health authorities reported two suspected cases of a mysterious inflammatory disease affecting children and teens linked to COVID-19 in Seoul on Tuesday.

This comes just a day after South Korea activated a system to monitor the illness known as pediatric multisystem inflammatory syndrome(PMIS). However, authorities said that neither case--one a teenager and the other under age ten--has tested positive for COVID-19.

Kwon Joon-wook, deputy director of the Korea Centers for Disease Control and Prevention, said the younger patient does not meet all the conditions for the illness, but both are being examined. 

PMIS, also known as multisystem inflammatory syndrome in children (MIS-C), has been spreading in the United States and Europe. As of Saturday, it has hit 13 countries since the first reported case last month.

The illness is similar to Kawasaki disease which causes inflammation in children aged four and younger. It's known to even lead to death in the worst case but the exact cause is yet unknown.