Officials are warning swimmers after two children were stung by a Portuguese man-of-war on Fire Island beaches.

Purple flags are flying at Islip Town beaches to warn beachgoers.

According to officials, a young boy was airlifted to Southside Hospital Tuesday after stepping on the venomous organism on a Kismet beach while playing. A short time later, a 4-year-old boy was stung at Davis Park.

The man-of-war can cause a painful sting that leaves red welts on skin for up to three days. In some cases, it can affect breathing or cause a heart condition.

Lifeguards say that they will be keeping caution flags up until there is no sign of the man-of-war off the South Shore. They say they will be inspecting the coastline every few hours.

Lifeguards say they believe the man-of-war are washing up from the Jersey Shore, where a few were found last week. They are not indigenous to Long Island.